A killer whale:

No dieting and always dressed for formal occasions.

I'm a beluga whale in the morning, a siren at night

One that don’t get eaten

Shark

Kraken

Otter. They are fast, fresh water, have sharp little teeth, good hunters, and they are cute and playful.

Chironex Fleckeri aka sea wasp aka extremely venomous box jellyfish.

Something with some type of defense mechanism, like a stingray or a jellyfish.

Something beautiful but barely sentient.

I think I would go with dolphin too.🐬

Either lobster or angler fish

I want a giant seahorse

I oughter be an otter

Octopus

Bull shark.

Orca

Octopus