Put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it. If you control a Merfolk, draw a card.
{2}{G}{U}
Arixmethes, Slumbering Isle
Legendary Creature - Kraken
Arixmethes, Slumbering Isle enters the battlefield tapped with five slumber counters on it.
As long as Arixmethes has a slumber counter on it, it's a land. (It's not a creature.)
Whenever you cast a spell, you may remove a slumber counter from Arixmethes.
{T}: Add {G}{U}.
12 /
12
{G}
Carpet of Flowers
Enchantment
At the beginning of each of your main phases, if you haven't added mana with this ability this turn, you may add X mana of any one color, where X is the number of Islands target opponent controls.
{2}{U}
Chasm Skulker
Creature - Squid Horror
Whenever you draw a card, put a +1/+1 counter on Chasm Skulker.
When Chasm Skulker dies, create X 1/1 blue Squid creature tokens with islandwalk, where X is the number of +1/+1 counters on Chasm Skulker. (They can't be blocked as long as defending player controls an Island.)
1 /
1
{1}{G/U}{G/U}
Cold-Eyed Selkie
Creature - Merfolk Rogue
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Cold-Eyed Selkie deals combat damage to a player, you may draw that many cards.
1 /
1
{5}{U}{U}
Colossal Whale
Creature - Whale
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Colossal Whale attacks, you may exile target creature defending player controls until Colossal Whale leaves the battlefield. (That creature returns under its owner's control.)
5 /
5
{1}{U}
Convincing Mirage
Enchantment - Aura
Enchant land
As Convincing Mirage enters the battlefield, choose a basic land type.
Enchanted land is the chosen type.
{1}{U}
Dream Thrush
Creature - Bird
Flying
{T}: Target land becomes the basic land type of your choice until end of turn.
1 /
1
{3}{U}
Dreamwinder
Creature - Serpent
Dreamwinder can't attack unless defending player controls an Island.
{U}, Sacrifice an Island: Target land becomes an Island until end of turn.
4 /
3
{3}{U}
Engulf the Shore
Instant
Return to their owners' hands all creatures with toughness less than or equal to the number of Islands you control.
{3}{U}
Ethereal Whiskergill
Creature - Elemental
Flying
Ethereal Whiskergill can't attack unless defending player controls an Island.
4 /
3
{5}{U}
Floodchaser
Creature - Elemental
Floodchaser enters the battlefield with six +1/+1 counters on it.
Floodchaser can't attack unless defending player controls an Island.
{U}, Remove a +1/+1 counter from Floodchaser: Target land becomes an Island until end of turn.
0 /
0
{U}
Genju of the Falls
Enchantment - Aura
Enchant Island
{2}: Enchanted Island becomes a 3/2 blue Spirit creature with flying until end of turn. It's still a land.
When enchanted Island is put into a graveyard, you may return Genju of the Falls from your graveyard to your hand.
{4}{U}{U}
Harbor Serpent
Creature - Serpent
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Harbor Serpent can't attack unless there are five or more Islands on the battlefield.
5 /
5
{7}{U}{U}
Inkwell Leviathan
Artifact Creature - Leviathan
Trample
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Shroud (This creature can't be the target of spells or abilities.)
7 /
11
{4}{U}{U}{U}
Jokulmorder
Creature - Leviathan
Trample
Jokulmorder enters the battlefield tapped.
When Jokulmorder enters the battlefield, sacrifice it unless you sacrifice five lands.
Jokulmorder doesn't untap during your untap step.
Whenever you play an Island, you may untap Jokulmorder.
12 /
12
{5}{U}{U}
Kraken of the Straits
Creature - Kraken
Creatures with power less than the number of Islands you control can't block Kraken of the Straits.
6 /
6
{1}{U}
Lingering Mirage
Enchantment - Aura
Enchant land
Enchanted land is an Island.
Cycling {2} ({2}, Discard this card: Draw a card.)
{1}{G}
Nylea's Presence
Enchantment - Aura
Enchant land
When Nylea's Presence enters the battlefield, draw a card.
Enchanted land is every basic land type in addition to its other types.
{1}{G}{G/U}{U}
Quandrix Cultivator
Creature - Turtle Druid
When Quandrix Cultivator enters the battlefield, you may search your library for a basic Forest or Island card, put it onto the battlefield, then shuffle.
3 /
4
{3}
Quicksilver Fountain
Artifact
At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land they control of their choice. That land is an Island for as long as it has a flood counter on it.
At the beginning of each end step, if all lands on the battlefield are Islands, remove all flood counters from them.
{U}
Reef Shaman
Creature - Merfolk Shaman
{T}: Target land becomes the basic land type of your choice until end of turn.
0 /
2
{2}{U}
Rootwater Commando
Creature - Merfolk
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
2 /
2
{5}{U}{U}
Scourge of Fleets
Creature - Kraken
When Scourge of Fleets enters the battlefield, 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.
6 /
6
{4}{U}{U}
Sea Monster
Creature - Serpent
Sea Monster can't attack unless defending player controls an Island.
6 /
6
{U}
Sea's Claim
Enchantment - Aura
Enchant land (Target a land as you cast this. This card enters the battlefield attached to that land.)
Enchanted land is an Island.
{3}{U}{U}
Sealock Monster
Creature - Octopus
Sealock Monster can't attack unless defending player controls an Island.
{5}{U}{U}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When Sealock Monster becomes monstrous, target land becomes an Island in addition to its other types.
5 /
5
{2}{G/U}
Selkie Hedge-Mage
Creature - Merfolk Wizard
When Selkie Hedge-Mage enters the battlefield, if you control two or more Forests, you may gain 3 life.
When Selkie Hedge-Mage enters the battlefield, if you control two or more Islands, you may return target tapped creature to its owner's hand.
2 /
2
{4}{U}
Serpent of the Endless Sea
Creature - Serpent
Serpent of the Endless Sea's power and toughness are each equal to the number of Islands you control.
Serpent of the Endless Sea can't attack unless defending player controls an Island.
* /
*
{5}{U}{U}
Slipstream Eel
Creature - Fish Beast
Slipstream Eel can't attack unless defending player controls an Island.
Cycling {1}{U} ({1}{U}, Discard this card: Draw a card.)
6 /
6
{4}{U}{U}
Spectral Deluge
Sorcery
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.
Foretell {1}{U}{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.)
{6}
Spire Golem
Artifact Creature - Golem
Affinity for Islands (This spell costs {1} less to cast for each Island you control.)
Flying
2 /
4
{3}{G}
Splendid Reclamation
Sorcery
Return all land cards from your graveyard to the battlefield tapped.
{1}{U}
Spreading Seas
Enchantment - Aura
Enchant land
When Spreading Seas enters the battlefield, draw a card.
Enchanted land is an Island.
{5}{U}{U}{U}
Stormtide Leviathan
Creature - Leviathan
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
All lands are Islands in addition to their other types.
Creatures without flying or islandwalk can't attack.
8 /
8
{1}{U}{U}
Streambed Aquitects
Creature - Merfolk Scout
{T}: Target Merfolk creature gets +1/+1 and gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)
{T}: Target land becomes an Island until end of turn.
2 /
3
{1}{U}{U}
Thada Adel, Acquisitor
Legendary Creature - Merfolk Rogue
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Thada Adel, Acquisitor deals combat damage to a player, search that player's library for an artifact card and exile it. Then that player shuffles. Until end of turn, you may play that card.
2 /
2
{U}
Tidal Warrior
Creature - Merfolk Warrior
{T}: Target land becomes an Island until end of turn.
1 /
1
{U}
Tide Shaper
Creature - Merfolk Wizard
Kicker {1} (You may pay an additional {1} as you cast this spell.)
When Tide Shaper enters the battlefield, if it was kicked, target land becomes an Island for as long as Tide Shaper remains on the battlefield.
Tide Shaper gets +1/+1 as long as an opponent controls an Island.
1 /
1
{U}
Tideshaper Mystic
Creature - Merfolk Wizard
{T}: Target land becomes the basic land type of your choice until end of turn. Activate only during your turn.
1 /
1
{2}{U}
Vedalken Plotter
Creature - Vedalken Wizard
When Vedalken Plotter enters the battlefield, exchange control of target land you control and target land an opponent controls.
1 /
1
--
Squid
Token Creature - Squid
Blue
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)