Sea Lantern

Sea Lantern is a light source that only appears underwater, giving off a light level of 15.

World Generation
Sea lanterns generate naturally in ocean monuments to light them. They can only be retrieved with a silk touch pickaxe, otherwise they only drop 2-3 prismarine crystals (up to 5 with fortune).

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.
 * In the preview image, sea lanterns were a purple\red shade instead of blue.
 * The sea lantern is one of the few blocks to have an animated texture, the others being prismarine, water, lava, Nether portal, and End portal.