Merfolk

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