Modelengine mobs:

Spermwhale:
uses coded sonar to locate giant squids and fights them, causes nausea if player is very close,
jumps above water with a splash, occasionally squirts water from blowhole above water
Hammerhead shark:
eats almost anything including humans, pulls the prey towards his jaws, has a chance to bite and leave
Giant squid:
hunts all fish and will fight spermwhales[/spoiler]

Single entity item mobs: (all of these are usable as statues and trophies)

Anglerfish:
hunts all fish, emissive
Halitrephes jellyfish
superstrong venom that can potentially kill player, emissive, kills all fish nearby
Nautilus
hunts small fish and crabs
Tuna
fast predator, hunts all small and medium fish
Sea star
seabed animal, food for others, eats corpses
Iron snail
seabed inhabitant, minds his own business[/spoiler]

Loot items:

small, large and medium fish meat
ambergris
tentacle
pile of shells
shark fin
potent venom
shark meat
whale meat