Crafting Recipe

PSP
SCS
PSP

P = dark prismarine; S = prismarine shard; C = Crystal

Ability

To use the Aqua Crystal, right-click it when it is in your hand. It will permanently (or until you drink milk) give you Conduit Power III and Dolphin's Grace III and will use up the Aqua Crystal

Obtainability

The most obvious source of the prismarine is from looting an Ocean Monument, which are sufficiently rare. Otherwise, you might get lucky with an Ocean Ruin, although the exact types of prismarine needed to craft this may not be present there.

Strategy

The Aqua Crystal essentially allows you to live in the water. It might be an interesting idea to make an underwater base or something of that sort. It also might be helpful when you are mining in underwater ravines or caves.