Sea Mobs

The glass fish is as it says, made of glass.
You can spot it in relatively deep waters (around 6/7 blocks needed for it to spawn) and it doesnt hurt you.
It is an Equipable helmet of sorts (like the turtles and bunnies) and it gives you the ability to breath underwater.
Seeming as the fish is glass, you stick it right on your head and can see perfectly fine and the fish filters the water while swimming. But only while swimming, once you get out of water, the fish starts strangling you so it can try and get back to the water.

The box jellyfish is a pink/blue/green/white/black blob in 2/3 deep water (like squid)
They resemble the shape of a squid except it has more tentacles that are "flowing" as well as the main body is more shorter and overall it is smaller in general.
It is passive most of the time but grows hostile quite easily.
It can grow hostile by you, making any kind of contact with water mobs (excluding glass fish for it is hard to see), removing a block of water via bucket or block, killing any mob.
It attacks by the main body grows longer and it's tentacles straighten, it also then turns towards you and fires a quarter block (size, not actual block) of silt that deals 1-6 hearts of damage (depending on colour).
It's size also varies depending on colour to show which would be more dangerous.