Atlantis is a mod that adds a completely underwater dimension that becomes available once you slay the 3 elder guardians in the ocean monument. The portal appears as a solid blue block right after they die. Simply right-click it to instantly teleport to Atlantis! But be careful not to destroy the block because you can’t craft it and there’s only one other way to generate one that isn’t easy either.
The whole dimension is filled with colorful reefs and shells as far as you can imagine, but don’t worry too much about drowning since the dimension automatically grants permanent water breathing, haste, and night vision!
There’s an alternate way to get to and from Atlantis if you don’t want to fight elder guardians and that’s achieved by crafting an Orb of Atlantis. You’ll need Atlantean crystals and Aquamarine gems so it isn’t cheap but once you have one, right-click on any block and a portal block will appear.