Sea Lantern

Sea Lantern is a light source that only appears underwater. The block gives off a light level of 15. The sea lantern has an animated texture.

Obtaining
Sea Lanterns can be crafted with prismarine shards and prismarine crystals, or found in ocean monuments. They are renewable, as crystals are obtainable as drops from Guardians and Elder Guardians.

As a light source
Sea lanterns emit a light level of 15, which is the brightest possible light level in the game. They are equal to glowstone, redstone lamps, jack o'Lanterns and beacons with the light they emit.

As a source of prismarine crystals
When broken using anything other than a Silk Touch-enchanted tool, a sea lantern will drop 2–3 prismarine crystals. A Fortune enchantment increases the maximum by 1 per level up to a maximum of 5, with Fortune III increasing the chance of larger drops. This method of obtaining Sea Lanterns is wasteful though; the 4 Prismarine Shards and up to 3 of the Prismarine Crystals used to craft it are non-retrievable.

Trivia

 * The sea lantern looks very similar to the block that appears to be a diamond block in the beacon, despite one not being used in the crafting.