msem-instigator

When Three Seas Squire enters the battlefield, target creature or Vehicle gets +2/+1 until end of turn.

Whenever Thundering Towershell attacks, return a creature card with mana value 5 or less from a graveyard to the battlefield under your control. Trigger a death knell of that creature.

Flash
Menagerie — When Tidal Trickster enters the battlefield, if there are five or more creature types among creatures you control, you may search your library for a card named Tidal Trickster, reveal it, put it into your hand, then shuffle.

Whenever Tide Crasher attacks, if you control another Merfolk, tap target creature defending player controls.

Experience (Whenever this creature attacks, if it has fewer than three +1/+1 counters on it, put a +1/+1 counter on it.)
When Tidepool Turtle has three or more +1/+1 counters on it, remove them all and transform it.

First mate — Each nonland card in your starting deck is an artifact, a Pirate, or has “artifact,” “Treasure,” or “Pirate” in its printed rules text. (If this card outside the game is your chosen first mate, you may swap it with a card in your opening hand.)
Whenever Tinbeard of the Brine attacks, each opponent loses X life and you gain X life, where X is the number of artifacts you control plus the number of Pirates you control.

Colossal 2—{6}{U} (You may cast this for its colossal cost to have it enter with two +1/+1 counters; or pay {6}{U} as a sorcery: Put two +1/+1 counters on it if it has none.)
When Titanic Fathomclaw's colossal ability puts one or more counters on it, return target instant or sorcery card from your graveyard to your hand.

Whenever Titanic Hardshell deals combat damage to a player, draw a card.
{2}{G}: Titanic Hardshell gains trample until end of turn.

Tizzanier Tidehunter can't be blocked as long as you control another Merfolk.

({T}: Add {U} or {R}.)
Torrid Geysers enters the battlefield tapped unless you control two or more basic lands.

Vigilance
When Tortle Guard enters the battlefield, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Trample
{4}{R}{R}: Tortle Tinkerer explores. This ability costs {1} less to activate for each artifact you control. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

{G}, {T}: Add one mana of any color. When a creature enters the battlefield this turn, if that mana was spent to cast it, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Tortle Traveler enters the battlefield tapped.
When Tortle Traveler enters the battlefield, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Whenever Tortle Veteran or another Turtle enters the battlefield under your control, that creature explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Haste
Whenever Tortle Waymaker becomes blocked, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

({T}: Add {W} or {U}.)
Tranquil Stream enters the battlefield tapped.

As long as you control three or more artifacts, Treasury Master gets +2/+0 and has slipstream. (This creature can't be blocked as long as only creatures with slipstream are attacking.)

When Trench Breaker enters the battlefield, put target creature on top of its owner's library.

Hexproof
Whenever this creature transforms into Tsunami Titan or attacks, you may return target creature to its owner's hand.

Hexproof

Respawn (When this creature dies, if it attacked this turn, return it to its owner's hand.)
When Tutorial Boss enters the battlefield, create a 1/1 blue Pirate creature token.

{1}{U}: Twilight Privateer voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)
Whenever Twilight Privateer phases in, up to one other target creature voyages.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
{1}{B}, Sacrifice a creature: Put a +1/+1 counter on Twisted Cultist.