Underwater Biome Mod 1.15.2/1.14.4 is added into Minecraft with the unique addition of new ocean biomes as well as structures and entities.
It is worth noting that you are able to see the mod as an adventure in which you will have to fight and defeat many powerful bosses and try to gather new weapons or accessory.
This time, it is a little bit the same as subnautica. You have to land on a mysterious planet with only water. In order to survive in this place, you need to collect the knowledge studied by crafting various recipes as well as discovering many different biomes. Additionally, you have to take advantage of the knowledge to unlock new recipes.

That is all about Underwater Biome. You can download it for free simply by a click.
How to install Underwater Biome Mod:
Assure that you have already installed Minecraft Forge
Define the location of the Minecraft application folder.
On the windows, open Run from the Start Menu, write %appdata% and click to Run
Open Mac on the finder, press and hold Alt and click to Library on the peak of the Menu Bar. Open the Application Support directory and look for Minecraft.
PutUnderwater Biome Mod you have already downloaded (.jar) into Mods directory.
When you start using Minecraft, open the mod button, you will realize that Underwater Biome Mod has been set up.