Cod, salmon, tropical fish, pufferfish and dolphins don't ride minecarts underwater or out of it, but other underwater mobs do like guardians, turtles and drowned.

How to reproduce:

1. Get a minecart and rail, then put them on the ground .

2. Try to spawn any kind of fish inside it.

3. You will notice that fish don't ride minecarts even if they pass through them.

Expected result: 

Every mob should be able to ride minecarts even underwater ones.