Flexible Waterbender

Vigilance
Waterbend {3}: This creature has base power and toughness 5/2 until end of turn. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)

Water Whip

As an additional cost to cast this spell, waterbend {5}. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)
Return up to two target creatures to their owners' hands. Draw two cards.

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.

Otter-Penguin

Whenever you draw your second card each turn, this creature gets +1/+2 until end of turn and can't be blocked this turn.

Katara, Bending Prodigy

At the beginning of your end step, if Katara is tapped, put a +1/+1 counter on her.
Waterbend {6}: Draw a card. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)

Yue, the Moon Spirit

Flying, vigilance
Waterbend {5}, {T}: You may cast a noncreature spell from your hand without paying its mana cost.

Geyser Leaper

Flying
Waterbend {4}: Draw a card, then discard a card. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)

It'll Quench Ya!

Counter target spell unless its controller pays {2}.

Iguana Parrot

Flying, vigilance
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Serpent of the Pass

If there are three or more Lesson cards in your graveyard, you may cast this spell as though it had flash.
This spell costs {1} less to cast for each noncreature, nonland card in your graveyard.

Flying Dolphin-Fish

Flying (This creature can't be blocked except by creatures with flying or reach.)

Deny Entry

Counter target creature spell. Draw a card, then discard a card.

Island

Giant Koi

Waterbend {3}: This creature can't be blocked this turn. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)
Islandcycling {2} ({2}, Discard this card: Search your library for an Island card, reveal it, put it into your hand, then shuffle.)

Lost in the Spirit World

Return up to one target creature to its owner's hand. Create a 1/1 colorless Spirit creature token with "This token can't block or be blocked by non-Spirit creatures."

Sokka's Haiku

Counter target spell.
Draw a card, then mill three cards.
Untap target land.

Rowdy Snowballers

When this creature enters, tap target creature an opponent controls and put a stun counter on it. (If a permanent with a stun counter would become untapped, remove one from it instead.)

Watery Grasp

Enchant creature
Enchanted creature doesn't untap during its controller's untap step.
Waterbend {5}: Enchanted creature's owner shuffles it into their library. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)

Waterbending Lesson

Draw three cards. Then discard a card unless you waterbend {2}. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)