Odd Water Mobs

The Anglerfish: It spawns quite often in deep oceans and is aggressive towards other smaller fishes. Its meat can be used to get a new light source.

The Coelacanth: It spawns very rarely and only when it's raining in deep ocean dunes. It can sell for a lot of emeralds.

The Isopod: A water arthropod that only spawns in the new biome. Its meat can be cooked or used in a new brewing recipe.

The Horseshoe Crab: Another water arthropod that spawns in beaches near the water. It drops its thorax, which can be used in another new brewing recipe.

The Giant Spider Crab: These passive crustaceans will spawn in cold oceans. They'll roam the seafloor and can dig up stuff from the sand if they're underwater. The stuff they dig up depends of the type of sand below them (more info here). They can breed if you feed them almost any meat, dropping crab eggs which you can use to spawn baby spider crabs. They drop crab legs which you can eat raw or cooked.

The Ghost Shark: A deep sea fish that comes in 4 different variants. They're venomous but passive creatures, and their meat can be used to get their stings, which are consumables that give a long lasting poison effect to other mobs.

The Mudskipper: These are small but territorial creatures that wander the swamps near water. They jump around to impress each other but they get alert if you get near them. You can also find blue alpha mudskippers. These try to fight any other alpha they find to protect their territory and If one ends up killing another they'll drop their fin.

Deep Sea Fish: These ambient fish live in the Deep Ocean Dunes and prey on each other. There are 4 different sizes and 11 different species in total, and they drop deep sea fish meat.

These species are: Sabertooth Fish, Velvet Whalefish, Grenadier Fish, Hatchetfish, Fangtooth Fish, Dragonfish, Telescope Fish, Viperfish, Lantern Shark, Flashlight Fish and Pearleye Fish.

Squat Lobsters: They are friendly and curious animals that come in 7 colorful variants. You can find each one on a different ocean biome and they drop their tail once dead.

List of the species here.

The Sea Pig: A rare sea cucumber that spawns on the bottom of the deep ocean dunes. If they spend around 30 minutes feeding over sand or gravel they'll secrete a slimy substance along with some particles. Then if you right click it, it'll give you the new Poison Protection effect for 15 minutes. You can also use its meat to try to get the effect by eating it raw (1 in 5 chance) or to brew a potion. If you're lucky you'll find one carrying a baby spider crab on its back.