RisenDeep

You should consider more team cards instead of playing for there own cards when focussing on Merfolk. Switching Metallic Mimic out for a 4 drop Deeproot Elite together with another 4 drop of Merfolk Branchwalker and Silvergill Adept should provide a first stable foundation. Then anything you do with counters is just hoping to get a Herald out? Why not use some Rishkar, Peema-Renegat to provide also an even better mana base instead or draw cards in emergency with Shapers of Nature? While playing for budget, maybe another Kopala could fit well into the deck.

On the spell base there are also Blossoming Defense and Adventurous Impulse I had good results with; oh and forgot Time of Ice, this small little saga puts a lot of preassure to your opponent :D

Deathrite Shaman has been banned in all formats except Vintage you know?
https://magic.wizards.com/en/articles/archive/news/july-2-2018-banned-restricted-update-2018-07-02

You need forests to play green unless I oversee something!? Some of Hinterland Harbor, Unclaimed Territory or even Memorial to Unitycould help here along with some Forest cards.
As card advantage, I like to play Adventurous Impulse in my standard brew along with Tempest Caller and Rescue.

Make it Vintage legal maybe!?

With all your legends, you play multiple styles in the deck that don't synergy that much.
Grand Warlord Radha is effective only with
- Enougth creatures to attack
- A spell in hand or enougth cards to benefit from the mana boost
Hallar, the Firefletched is effective only with
- A lot of Kicker spells - or -
- Enougth counters to benefit from just one Kicker spell
Marwyn, the Nurturer is effective only if
- She was played before any other Elf creature

--------------------------------------
You could make this deck standard legal, cut anything with Kicker except for 1 or 2 Fight with Fire and instead set on counters on your legends. Some usefull spells are:

Song of Freyalise as it works automatically and your creatures also are able to produce mana.
Kujar Seedsculptor because it is also an Elf.
Ancient Animus, that also works well as creature removal when you add counters to your legendaries.
Arbor Armament to block creatures with flyng.
Durable Handicraft is a well working booster card for your creatures and also able to give any of them additional counters.

--------------------------------------
Less effective spells are:
Enter the Unknown because it is gambling if you get a counter or another mana this turn.

--------------------------------------
Then I would play for aggro/ramp using your legends like Radha, Marwyn and Rishkar to grow and power your pack while Hallar should just be in a secondary role to be powered and combating first before playing Fight with Fire as finisher card. Use any mana ramp and protector spell you could get and cards like Adventurous Impulse to find those cards you need in your deck. Armorcraft Judge is a good drawer card and also an Elf so you would benefit twice.

And banned too in modern

I played this with Stonehewer Giant as replacement for Stoneforge Mystic in modern. The only disadvantage is that he is {2}{w} more expensive than Mystic

I would replace Benefaction with Adventurous Impulse. Impulse is cheaper with {G} than the Ronas spell

In Modern you have a standard configuration that consist usually of
- Cursecatcher, to protect your Merfolk from spells
- Silvergill Adept, to draw cards
- Harbinger of the Tides, to remove nasty creatures from the board
- Merfolk Trickster, to skip abilities of creatures
- Lord of Atlantis, Master of the Pearl Trident, to buff your creatures and give'em Ilandswalk
- Merrow Reejerey, more power and the ability to tap/untap whenever you play a Merfolk spell
- Kopala, Warden of Waves, Kira, Great Glass Spinner to prevent your creatures from being targeted by spell and abilities
- Master of the Waves, as finisher, while he has protection against red and can't be removed by red spells, abilities and red damage, he also generates an army of elemental tokens that may end the game next turn

- Aether Vial, to bring your creatures onto the board for free
- Spreading Seas, Sea's Claim, to make islands for the lords ability
- Dismember, to remove a disturbing creature from the board

there are also builds that utilize
- Kumena's Speaker, because you have a T1 2/2 creature (maybe)
- Merfolk branchwalker, because it acts similar to Silvergill Adept
- Merfolk Mistbinder, as an additional lord without Islandwalk

in the old standard meta from 2009, there were also build playing
- Merfolk Sovereign, because Lord of Atlantis rotated out of the standard format
- Deepchannel Mentor, makes your creatures unblockable to finish the game

I also prefered some experimental builds splashing white/red using also
- Lightning Bolt
- Blood Moon
- Chalice of the Void
- Sygg, River Guide
- Path to Exile

Yes it is, unless you loose the game :D

Could work with a lot more of deck searching cards. Maralen, Demonic Tutor, Vampiric Tutor, Bseech the Queen, or even Doomsday

Deepchannel Mentor is overkill because nearly any blue card you play is always unblockable in some way while you play more black cards as blue ones.
Oona's Blackguard is set and Sygg, River Cutthroat too while Oona's Prowler prowler is also a hard bank you should play.
Thieving Sprite could be a good additional hand disruptor card while Riptide Pilferer could also do the job.
Thada Adel, Acquisitor is a nice card if your opponents play some artifacts you could utilize when stolen.
Waste Not is a very good supporter card for rogues because of the number of discard and hand disruption abilities or you use something similar to Liliana's Cares

I cant imagine that Tatyova generates so much value here that it earns a 2 drop in main. Also I would put Kopala in, either as a 1 drop replacement for Tatyova or into your board

+1 for using Tatyova

Have it in stock but not played yet unless in Tiny Leaders Merfolk. I feeled having it in hand waiting all the time for the right card to drown into spoil but as I've played arround 100 games with it yet, cant think if it would worth putting it into the modern meta.

Even if I think that its ability to flash for free is a pro against Harbinger of the Tides extra cost of 2.

Maybe depend on the situation where Harbinger benefits over Trickster and vice versa. Against Tron and Scapeshift Trickster will be the better option to prevent a Primeval Titan from shifting lands into the game or killing Wormcoil while Tron has enougth resources to play it again if you simply flash it back to there hands

Ah no!
Simply No!

First, no one would play Kopala 4 times in a deck, that is insane because she is legendary so cutting two sounds like a good plan. Second, High Tide, really? High tide is a storm card in combo with untappers like Unwind or Cloud af Faeries but it hasnt any space in merfolk tribal deck, even for a beginner. Same for Mysteries of the Deep, 5 mana to draw 2 cards is insane!

Aquatic Incursion and Deeproot Waters are not worth playing them as 4 drop either. The one dosent benefit from the other and both have little value when not playing enougth merfolk spells. Not to mention that Merfolk Sovereign herself already makes creatures unblockable right?

So now your creature base, where is Silvergill Adept? It is the best and cheapest draw + hitter card you have for your merfolk tribal deck and costs even a few cents as same as Merrow Reejerey. Cut Merfolk Mesmerist and Merfolk Spy and stock Cosi's Trickster up to a full playset will help you as same as cutting the Architects for something like Spreading Seas and add a few cheap removal and/or counter spells to your deck. Unsummon and Mana Leak or Unsubstanciate would do the job quiet well.

I wrote a long Merfolk Budget Primer some time ago, maybe you take a look at this
https://www.mtgvault.com/risendeep/decks/merfolk-bdg/

Beginner dosent mean that the person needs to play a bad designed deck!

Deeproot Champion cant really benefit from thid deck, rather replace it with Deeproot Elite

Wont play it like that but let me explain:

Deeproot Elite and Merfolk Mistbinder are good options even while the one dosent benefit from the other. This means Mistbinder "just gives" +1/+1 while Elite adds counters but ok, a solid foundation. I would also add Metallic Mimic if you can get your hands on a playset

Forerunner of the Heralds seems way to expensive for what he is doing. Merfolk is not as powered in standard as it is in modern so you should be fast as hell to beat any other deck in the format.

Kopala, Warden of Waves is a good choice too for end game protection, you should test if a 2 drop is enougth or should be 3 ones. An alternative could be Swift Warden.

Jade Bearer should be set as 4 drop too because it powers your guys additionally and itself provides an additional body. Works well in combination with Deeproot Elite.

Jungleborn Pioneer could help getting more bodies to beat down your opponent and it also profits from Deeproot Elite, both Pioneer and the token.

Merfolk Branchwalker should be double paired with Silvergill Adept, while Merfolk Branchwalker explores Silvergill Adept will draw cards. Both has advantages on its own and both should be played.

Seafloor Oracle can help fetching whatever you need in late game.

Dont play Admiral's Order, this spells alternate casting cost may be not triggered as often as you would like to and you cant use it in early turns when not attacking. Instead focus on Blossoming Defense, a full playset should work here.

Deeproot Waters dont give that benefit you need. You need to cast it turn 3 and play an additional guy turn 4, to get a 1/1 token. You can spend your mana to other spells instead so cut that

Jace, the Mind Sculptor was banned until this year

Playtesting pointed out that there was some misbehavior in the mana base and some bad drops that let fail the game. Adjusted the mana base so it works better now with the black components while still working with the green ones

You know that the spell with Flashback gets exiled so you still need another Glimpse the Unthinkable in hand what makes blinking Snapcaster something of redundant