RisenDeep

Looks neat on a first view! I personally (as Merfolk player from first hour) would play 4 Aether Vial main because they are a better option in late game than Cavern of Souls are. Hate cards are as same for lands as for artifacts so there would be no difference here but Aether Vial in late game for 2 or 3 counters will save a lot mana to spend and you could flash ANY spirit on the board (having a Aether Vial on 1 and a Cursecatcher in my backhand saved me a lot of games for instant counter important stuff).

I think Kira is a bit of shooting with canons on birds because nearly each spirit you play either has Hexproof on its own or gets Hexproof by another one.

But PliNter is also right for your powering compared to Merfolk (as you mentioned in your description). While Merfolk has its lords there is a little what spirits have. Battleground Geist is too expensive for its single +1/+0 powering, Gallows Warden is the same for thoughtness and at least Drogskol Captain as your one and only option while the clue about bant spirits is they have the Exalted ability for even more power. One option could be Phantasmal Image to copy clonw your Drogskol Captain but as nearly all of your spirits fly, you could also give Favorable Winds a try as I did for my Faeries (because they also lack for enougth lords)

As from experience, 4 Path to Excile is too much, 3 would also be enougth unless you play against decks theres key card is a pathable creature.

Your board cards seem pretty standard except I would replace Stony Silence for some Kataki, War's Wage because it is a spirit too and artifact heavy decks will not have enougth resources to pay for it

At the end this depends on the stack order if successfully or not ;)

Emrakul has but not the Egg

"Top Tier" might be a bit understated because you are quite high on mana here and realy prone to early game hate like Lightning Bolt and Fatal Push. A 2 for tap is a little bit expensive in early game too. I would reduce the high mana cards for cheaper ones, Mana Leak for example.

I wrote a small primer about that topic you might think usefull
http://www.mtgvault.com/risendeep/decks/merfolk-bdg/

It seems very unstable to get Emmy out at least for some exile like Path to Excile or damage spells with exile like Incinerate. Might be a try to look on Proteus Staff to get Emrakul on the board

At least when thought about "how get artifatcs into grave heavily", I came across this deck type/approach as a possible fast candidate. It is inspired by but modified to Scrapyard Salvo as its primary and Storm/Slag Fiend as its secondary win conditions with a little bit of Blood Moon

Agree; Added additional tag so one might see it more easily

To the spear and the splash, spear is cheap (in cost) and as I have had a long going deck like these that spalshed white a lot for utilizing Path to Exile and Sygg, River Guide

http://www.mtgvault.com/risendeep/decks/merfolk-2/

I could tell from experience that you will have two whites out for long or short and it is also a one drop so might play as well. The way I decided for Spear also was its side effect. Running 3 Memory Leak is an ok cut but condemn is not as good as Path to Exile I think so a little bit extra disruption would be nice here. You also oversee for Pyramid of the Pantheon that could produce mana additionally in any color so even hard cast Dismember is possible.

But the main reason to splash was to feed Sygg because it is a totally underrated card that saved me a lot of games against Jeskai Control, Infect Ramp and even Living End

What I could imagine would be additional plus cards but make the deck even more complicated are Dakmor Salvage and Bloodghast. You use Raven's Crime so having a land with Dredge in your graveyard could support casting it from even there and Bloodghast returns from graveyard too even when eating up some of your opponents removal spells.

I also would rely less on Sea's Claim rather than Evil Presence because most decks rely on blue

Tell me what the results of investigation were please so I could maybe try this on a tourney :)

It is a primer on budget merfolk so at the time I wrote this there were some cards available as replacement for expensive ones mentioned in deck description and I described there pros/cons and why I choose them for this variant of budget deck. Prices may change and if for example Aether Vial raises a top score on low pricing then this might be ok but for now it isnt and this deck is an image of current prices and market situation. You should not read "something about" a certain card but read the pointing paragraph about what, when and why ;)

Spear of Heliod is just another booster card that gives your fish an additional +1/+1 and has the second advantage to destroy creatures when in need of. It has found its way into this for the reason of the White splash, no more, no less

It seems very mana intensive and might be a little too slow due to Ixalan's tribal support in standard. You might add either more hate cards or a few mana boosters to get your win condition out. A Nissa Planeswalker could be a help here using its second ability along with some scry you might get lands our faster, also a card (I suggest very often last time) is Pyramid of the Pantheon as an extra 3 mana boost when fully charged.

Overflowing Insight seems a bit too expensive to me. You should use more instant cards like Opt or Pull from Tomorrow

Tree in combination with Diminish is an instant hit and there is enougth hit with Bump in the Night or Crackling Triton that makes the last shot anyways

I play in my deck

1 Dwynen, Gilt-Leaf Daen
3 Elvish Champion
3 Elvish Archdruid
4 Imperious Perfect
4 Joraga Warcaller

to power my elves. Your versions seems a bit stitched to your approach to put raw power in it. Oran-Rief, the Vastwood and Joarag Warcaller build a good combo so that you could get your first power boost in turn 2 instead of 3. Joarag Warcaller needs to be kicked for 2 but with Oran-Rief, the Vastwood you could place a counter on it when playing so you get a 1 mana discount here. Gavony Township gives boost in late game while placing counters on Joraga Warcaller leads to additional boost. Metallic Mimic gives additional counters when playing elf cards.

For mana aspect I added Joraga Treespeaker to have extra two mana in turn 2 and its last ability to make al elves mana boosters and a Viridian Joiner that gives mana egual to its power.

Elvish Champion and Lush Growth are also a good finishing combo when giving your opponent a forest

I personally would go for 22 lands first and as it seems that you want to go for green, just use some mana boost cards (SEarch your library for <Land> card ...) so you would have a solid mana base in the front and then go for synergy. Synergy means that cards set up on each other so for example Oran-Rief Hydra's landfall ability with any mana fetcher like Natural Connection to trigger Landfall an additional time this turn.

Go simple and dont try risky card combos that you cant protect against interruption. The simpler your win conditions are the more effort you will get where more complex win conditions (like ugly card combos) might be affected by holes in your plan when for example you rely on a creature to be on the battlefield but an opponent casts a removal spell on it

Yeah different playgrounds. We here have anything from Path to Exile up to Lightning Bolt and cheap (mana) bouncies like Vapor Snag to go for

Inspired by your deck
http://www.mtgvault.com/risendeep/decks/mangrovefish/

Quicksilver Fountain may potentially be too slow for fast games and not impacting enougth for late game as it effectively goes for Turn 3-4 (depending on starting player) earliest and you are completely tapped out then so a simple artifact hate spell will have you waste 3 mana. Even when your opponent cant do anything against it, a Blood Moon is a more heavy impact and you wont have any benefits from additional islands on your opponents side. Merfolk or something else with Islandwalk would profit way more from Quicksilver Fountain.

Dont mistake this for a total rejection critics; I like the idea anyways

It might work due to the suprise effect but should consider that decks are potentially full of creature hate cards

Vampires as well as Zombies seems pretty made for this kind of deck, however, Vampires have day one build in sacrifice mechanic with Bloodghast, Viscera Seer and Kalastria Highborn utilized with a lot of fetchlands. This may be very expensive in money.

It feels as you have too much combo pieces in your deck and too less control elements that may lead to your deck feeling janky

Seems fair ;)

I would remove all these mana expensive stuff and instead put more booster cards to get your fish stronger. Deepchannel Mentor was a secret tip card in Standard for when it was legal but makes no sense in Modern since it will come too late in the game to bring its benefits. Same for Waterspout Weavers as there benefits are too less for beeing worth the 5 mana. Stonybrook Banneret has some benefits with its ability but is way to easy drop for any removal spell. You do not need it when you play cheaper cards.

Mausoleum Wanderer is a totally mispick because it has absolutely no synergy with all the other cards except itself so why not use the good old Cursecatcher here?

Sage's Dousing was also a great card in lorwyn standrad but there are more reliable counter spells out there in modern and you might have enougth card draw to go for so I would drop it for other slots.

First, what makes a good merfolk deck as same as every other tribal is synergy between the cards and merfolk at its own has the benefits of having a wide range of synergy not only as an aggro kind of deck but also as mill, control, weeni, even token spitting is one of the facettes playing merfolk has. You might want to go for an Aggro deck as it seems from your list so I will focus on this.

The most important card is the, yeah expensive, Aether Vial. One may argue for Cavern of Souls but I have played well without it several years and matches but Aether Vial is pretty standard. I see from your comments you do not have the resources to grab a playset so we must offset it anyways. Its main benefits is to bring creatures on the board for free so we need something that could at least boost our creature outcome a little. I have to options that might be get a thought on

Springleaf Drum -> This card costs as much as Vial and generates one mana when used. The downside is that you need already a creature on the board to use it but a benefit is that you could use it also to make casting counter spells more easy.

Pyramid of the Pantheon -> This card costs as much as Vial but is bit tricky to use. You need to power it first before you could use it but at the point it is completely powered you benefit from 3 extra mana per use so might give it a try. Powering costs 1 mana effectively because you will get one mana back for its first ability so you might could consider use it when you have mana left (for example cast a counter spell for 2 in Turn 3).

Anything else seems to expensive in mana or to unreliable to me so I would tend to try the Pyramid option for vial.

Cursecatcher is also an important card to merfolk anyways, while it controls early game you have it grown up in the late game to help beating down your opponent. It is a must have

Lords are the core party of an merfolk aggro deck. They give you the power to boost your troops and getting game. There are Lord of Atlantis, Master of the Pearl Tridant as the expensive ones but also Coralhelm Commander, Merrow Reejerey and Merfolk Sovereign as cheaper more mana costing ones but when playing with a lot of wizard subtypes like Cursecatcher, Sovereign and Silvergill Adept you could also use such cards as Sage of Fables. This passes a +1/+1 counter on each wizard that enters the battlefield or Vineshaper Mystic from Ixalan also putting counters on your creatures.

Silvergill Adept is also a standard card used as a free drop because it gives you an extra card when playing (not entering the battlefield) and its 2/1 hit okay in early game.

So know we have the base just take a look at some extra drops. A good synergy card and propably budget replacement for Kira, Great Glass Spinner is Kopala, Warden of Waves from Ixalan. Spells and abilities will cost an extra 2 when targeting one of your merfolk cards in play. It is not an instand counter as from Kira but Kopala is also a merfolk so will benefit from your lords too.

Harbinger of the Tides is also a comon drop in merfolk meta play. Does not need any explanation here ;)

Some non-merfolk drops are Spreading Seas and Sea's Claim where I'm going to run not more than 2 copies of either or in my main decks while most modern competitive decks go with blue or blue splash so it might not be needed that much. Primary target was having a target for your lords to take a benefit from Islandwalk but are also able to impact on your opponents lands (when not playing for mono blue) and make non-baiscs to islands too (and islands have no abilities :D )

Now splashing; as a budget deck you would need any support you could get so splash for White or Green has additionally benefits- Sygg, River Guide is one of my favourite merfolk legends for white because it not only protects against removal spells but also can make your merfolk unblockable (by creatures of the choosen color). Green has such cards as Collected Company that helps fire more merfolks on the board but also upcomming Ixalan carries some green merfolk benefitters.

As writing these comment I will use this to create an own budget list here so you may take a look at this too ;)
http://www.mtgvault.com/risendeep/decks/merfolk-bdg/

Hope it helps