Dicks... All of them

Besides sharks like the other commenter said, there's a type of fish called "livebearers" who do indeed fuck and internally inseminate

They give birth to fully formed babies vs laying eggs and fertilizing them externally. A fascinating animal that falls in the category is the Amazon molly which gives birth to clones of itself

Sharks fuck. It's really unpleasant. They are also fish.

Yea. These fish fuck.