The Watcher in the Water

The Watcher in the Water enters tapped with nine stun counters on it. (If a permanent with a stun counter would become untapped, remove one from it instead.)
Whenever you draw a card during an opponent's turn, create a 1/1 blue Tentacle creature token.
Whenever a Tentacle you control dies, untap up to one target Kraken and put a stun counter on up to one target nonland permanent.

Scourge of Fleets

When this creature enters, return each creature your opponents control with toughness X or less to its owner's hand, where X is the number of Islands you control.

Harbinger of the Tides

You may cast this spell as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
When this creature enters, you may return target tapped creature an opponent controls to its owner's hand.

Chasm Skulker

Whenever you draw a card, put a +1/+1 counter on this creature.
When this creature dies, create X 1/1 blue Squid creature tokens with islandwalk, where X is the number of +1/+1 counters on this creature. (They can't be blocked as long as defending player controls an Island.)

Hullbreaker Horror

Flash
This spell can't be countered.
Whenever you cast a spell, choose up to one —
• Return target spell you don't control to its owner's hand.
• Return target nonland permanent to its owner's hand.

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Aether Channeler

When this creature enters, choose one —
• Create a 1/1 white Bird creature token with flying.
• Return another target nonland permanent to its owner's hand.
• Draw a card.

Wizard Class

(Gain the next level as a sorcery to add its ability.)
You have no maximum hand size.
{2}{U}: Level 2
When this Class becomes level 2, draw two cards.
{4}{U}: Level 3
Whenever you draw a card, put a +1/+1 counter on target creature you control.

Astral Dragon

Flying
Project Image — When this creature enters, create two tokens that are copies of target noncreature permanent, except they're 3/3 Dragon creatures in addition to their other types, and they have flying.

Man-o'-War

When this creature enters, return target creature to its owner's hand.

Twenty-Toed Toad

Your maximum hand size is twenty.
Whenever you attack with two or more creatures, put a +1/+1 counter on this creature and draw a card.
Whenever this creature attacks, you win the game if there are twenty or more counters on it or you have twenty or more cards in hand.

Serum Visionary

When this creature enters, draw a card, then scry 2.

Cloud Key

As this artifact enters, choose artifact, creature, enchantment, instant, or sorcery.
Spells you cast of the chosen type cost {1} less to cast.

Spirit Water Revival

As an additional cost to cast this spell, you may waterbend {6}. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)
Draw two cards. If this spell's additional cost was paid, instead shuffle your graveyard into your library, draw seven cards, and you have no maximum hand size for the rest of the game.
Exile Spirit Water Revival.

Hydro-Man, Fluid Felon

Whenever you cast a blue spell, if Hydro-Man is a creature, he gets +1/+1 until end of turn.
At the beginning of your end step, untap Hydro-Man. Until your next turn, he becomes a land and gains "{T}: Add {U}." (He's not a creature during that time.)

Aether Adept

Wavebreak Hippocamp

Whenever you cast your first spell during each opponent's turn, draw a card.

Naiad of Hidden Coves

During turns other than yours, spells you cast cost {1} less to cast.

Katara, Waterbending Master

Whenever you cast a spell during an opponent's turn, you get an experience counter.
Whenever Katara attacks, you may draw a card for each experience counter you have. If you do, discard a card.

Minn, Wily Illusionist

Whenever you draw your second card each turn, create a 1/1 blue Illusion creature token with "This token gets +1/+0 for each other Illusion you control."
Whenever an Illusion you control dies, you may put a permanent card with mana value less than or equal to that creature's power from your hand onto the battlefield.

Volatile Stormdrake

Flying, hexproof from activated and triggered abilities
When this creature enters, exchange control of this creature and target creature an opponent controls. If you do, you get {E}{E}{E}{E}, then sacrifice that creature unless you pay an amount of {E} equal to its mana value.

Glen Elendra Pranksters

Flying
Whenever you cast a spell during an opponent's turn, you may return target creature you control to its owner's hand.

Island

Illusionist's Gambit

Cast this spell only during the declare blockers step on an opponent's turn.
Remove all attacking creatures from combat and untap them. After this phase, there is an additional combat phase. Each of those creatures attacks that combat if able. They can't attack you or planeswalkers you control that combat.

Fatespinner

At the beginning of each opponent's upkeep, that player chooses draw step, main phase, or combat phase. The player skips each instance of the chosen step or phase this turn.

Defiler of Dreams

Flying
As an additional cost to cast blue permanent spells, you may pay 2 life. Those spells cost {U} less to cast if you paid life this way. This effect reduces only the amount of blue mana you pay.
Whenever you cast a blue permanent spell, draw a card.

Exclusion Mage

When this creature enters, return target creature an opponent controls to its owner's hand.

Nezahal, Primal Tide

This spell can't be countered.
You have no maximum hand size.
Whenever an opponent casts a noncreature spell, draw a card.
Discard three cards: Exile Nezahal. Return it to the battlefield tapped under its owner's control at the beginning of the next end step.

Shrieking Drake

Flying
When this creature enters, return a creature you control to its owner's hand.

Aquamoeba

Discard a card: Switch this creature's power and toughness until end of turn.

Cryptoplasm

At the beginning of your upkeep, you may have this creature become a copy of another target creature, except it has this ability.

Breath of the Sleepless

You may cast Spirit spells as though they had flash.
Whenever you cast a creature spell during an opponent's turn, tap up to one target creature.

Body of Knowledge

Body of Knowledge's power and toughness are each equal to the number of cards in your hand.
You have no maximum hand size.
Whenever this creature is dealt damage, draw that many cards.

Voracious Bibliophile

Flying, vigilance
Whenever you cast a spell with one or more targets, draw that many cards.

Geyser Drake

Flying
During turns other than yours, spells you cast cost {1} less to cast.

The Blue Spirit

You may cast the first creature spell you cast each turn as though it had flash.
Whenever a nontoken creature you control enters during combat, draw a card.

Alandra, Sky Dreamer

Whenever you draw your second card each turn, create a 2/2 blue Drake creature token with flying.
Whenever you draw your fifth card each turn, Alandra and Drakes you control each get +X/+X until end of turn, where X is the number of cards in your hand.

Mischievous Mystic

Flying
Whenever you draw your second card each turn, create a 1/1 blue Faerie creature token with flying.

Bender's Waterskin

Untap this artifact during each other player's untap step.
{T}: Add one mana of any color.

Faerie Mastermind

Flash
Flying
Whenever an opponent draws their second card each turn, you draw a card.
{3}{U}: Each player draws a card.

Ravenform

Exile target artifact or creature. Its controller creates a 1/1 blue Bird creature token with flying.
Foretell {U} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Nerd Rage

Enchant creature
When this Aura enters, draw two cards.
Enchanted creature has "You have no maximum hand size" and "Whenever this creature attacks, if you have ten or more cards in hand, it gets +10/+10 until end of turn."

Tempest Caller

When this creature enters, tap all creatures target opponent controls.

Bellowing Crier

When this creature enters, draw a card, then discard a card.

Mystic Reflection

Choose target nonlegendary creature. The next time one or more creatures or planeswalkers enter this turn, they enter as copies of the chosen creature.
Foretell {U} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Curiosity Crafter

Flying
You have no maximum hand size.
Whenever a creature token you control deals combat damage to a player, draw a card.

Triskaidekaphile

You have no maximum hand size.
At the beginning of your upkeep, if you have exactly thirteen cards in your hand, you win the game.
{3}{U}: Draw a card.

The Unagi of Kyoshi Island

Flash
Ward—Waterbend {4}. (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {4}. They can tap their artifacts and creatures to help. Each one pays for {1}.)
Whenever an opponent draws their second card each turn, you draw two cards.

Mulldrifter

Flying
When this creature enters, draw two cards.
Evoke {2}{U} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

Ghostly Pilferer

Whenever this creature becomes untapped, you may pay {2}. If you do, draw a card.
Whenever an opponent casts a spell from anywhere other than their hand, draw a card.
Discard a card: This creature can't be blocked this turn.

Kefnet's Monument

Blue creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, target creature an opponent controls doesn't untap during its controller's next untap step.

Homunculus Horde

Whenever you draw your second card each turn, create a token that's a copy of this creature.