Snap

Return target creature to its owner's hand. Untap up to two lands.

Aeromoeba

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

Bender's Waterskin

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

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.

Wavebreak Hippocamp

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

Aquamoeba

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

Harbinger of the Seas

Unwind

Counter target noncreature spell. Untap up to three lands.

Fierce Guardianship

If you control a commander, you may cast this spell without paying its mana cost.
Counter target noncreature spell.

Sapphire Medallion

Blue spells you cast cost {1} less to cast.

Eluge, the Shoreless Sea

Eluge's power and toughness are each equal to the number of Islands you control.
Whenever Eluge enters or attacks, put a flood counter on target land. It's an Island in addition to its other types for as long as it has a flood counter on it.
The first instant or sorcery spell you cast each turn costs {U} (or {1}) less to cast for each land you control with a flood counter on it.

Frantic Search

Draw two cards, then discard two cards. Untap up to three lands.

Divert Disaster

Counter target spell unless its controller pays {2}. If they do, you create a Lander token. (It's an artifact with "{2}, {T}, Sacrifice this token: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.")

Rewind

Counter target spell. Untap up to four lands.

Kozilek, Butcher of Truth

When you cast this spell, draw four cards.
Annihilator 4 (Whenever this creature attacks, defending player sacrifices four permanents of their choice.)
When Kozilek is put into a graveyard from anywhere, its owner shuffles their graveyard into their library.