Drakefish

Passive fish mobs that spawn in deep ocean biomes. They are black fish with a red spot on both sides of its body. They are about as big as full-grown salmon, if not a little bigger and thicker. Although they are passive, they have the thorns effect, so anything that hits them also takes some damage. Axolotls will not hunt them, this is to prevent unfortunate pet accidents. 

Upon death they drop their scales. Drakefish scales can be used to create aquatic arrows. Aquatic arrows are special arrows that function well in water, but poorly above water. Drakefish scales can also be used to craft a new block that can be used as an underwater cactus, as in it hurts anything that touches it, except drakefish. They could also drop their meat as food. 

Also just as a note, I was originally thinking of the name dragonfish, but forgot that those are real animals that are nothing like the fiction creature I came up with. And dragonfish was such a cool name too ):