Abyssal Depths is a port of the mod Aquatic Abyss
This mod adds sea life to the (pre-1.13) barren oceans of Minecraft, some based on real animals and others on mythical ones, and adds seaweed and rock formations. These features will liven the oceans for a much more interesting underwater experience.
Credit for the mod's name goes to mothmankai
Below are links to the CurseForge page of the original as well as the Minecraft Fourm page. The fourm page contains most of the information you will need to know about this mod, but make sure to read the changelog of the versions released here to see any changes I make.