something i think would be fun to add later in the game for water is mantis shrimp, maybe as a boss or an ambush predator
there are over 400 different species but if you only add two it will bring future biodiversity to the game but also colourful new weapons and the ability to use multiple synergies under the water
they are on average 10cm large (the larger ones mentioned can grow up to 5 inch or 7 for the pea**** while the smaller ones grow up to 3.5 inches) which is why given the size of our character I would make it as a baby variant or a boss. if going with the boss idea there could be 2 different types (explained later). because ominant wanted to see if the taming system could allow them to make species larger and more independent than they were able to work together, however this didn't work because their advances seemed like threats causing them to be killed leaving variations of their respective meals to be left over in notes on their bodies.
the reasons for this is in the first game we only really had the daggers to use in water however these creature (depending on the species) hunt/defend themselves in different ways such as the most popular way in media being smashers with a quick punch can break shells off crabs (like a barbarian synergy) such as the pea**** mantis shrimp (or a smaller variation being the purple spotted mantis shrimp). Skewer types (like a javeliner or use the raptor like blade for teir4 daggers) that hide in the seabed and punch upwards to pierce fish, an example of this is the yellow manta shrimp or a smaller variation being then pseudosquilla richeri (i couldn't find its none scientific name)).