Amphibious. The swarm can breathe air and water.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny beast. The swarm can't regain hit points or gain temporary hit points.

Claw. Melee Weapon Attack: +4 to hit, reach 0 ft., one target in the swarm's space. Hit: 10 (4d4) bludgeoning damage, or 5 (2d4) bludgeoning damage if the swarm has half of its hit points or fewer.

Rave. The swarm breaks into a crab rave until they decide to stop. This accomplishes nothing.