If you want to see new sea creatures in the game and not ordinary fish, but something more interesting, then install the Siren mod for Minecraft, which will add predatory mermaids to the game. These creatures will inhabit all the oceans and seas of the world, they will mainly live at depths from 0 to -45. These creatures are very aggressive and will attack both other underwater inhabitants and the player. If you get closer than 32 blocks to them, they will start attacking you.
In the water, the Sirens make sounds very similar to songs that can be heard 50 blocks away. If you are not ready to meet these predators, then always listen and if you hear a song, swim in the opposite direction.
In addition to the mobs themselves, a new structure will appear in the game, namely the temple of the sirens, where you can find a lot of useful and rare resources. This structure will appear on the surface of the ocean and will be quite difficult to find.