Marit Lage, the Ur-Tide

Flying, indestructible, convoke
Marit Lage, the Ur-Tide can't be sacrificed.
When Marit Lage, the Ur-Tide enters the battlefield, if you cast it from your hand, put a submerge counter on all other permanents.
Permanents with submerge counters on them are Islands with "If a creature would enter the battlefield under your control, sacrifice it unless
it is blue or has flying."