Just like "Land Creatures", Aquatics also have Hybrids.
These hybrids help your super rare aquatics surpass the stats the they currently had. Although there are not a lot of them, here are some you can look at:
It would be best to get Dakoderma as one of your very first aquatic hybrids in the game. Decently cheap, it should help you if you are struggling to keep up the power in your cavern class.
Leptostega should also e fused as soon as possible to help you have much, much more power in the reefs.
Good luck unlocking these hybrids as some of them would take longer.