The first step to unlocking Aquatic Power is to locate an ocean and to search for a small and unlikely structure.
The structure is mainly built out of dark prismarine with a mysterious device in the middle of the "arch" However, this structure should not be your main point of interest as below it, in a small radius, there often is a geode like cave, containing a mighty crystal.
Around the crystal there is also a variety of loot. The crystal itself is surrounded with scaffolding which might be useful later on...
Warning: Do NOT break the crystal (yet) as it seems to shatter completely, without leaving anything behind.
Next: Laser Power