Sea Lantern

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

Obtaining
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 amount of prismarine crystals dropped. 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.

Natural generation
Sea lanterns generate naturally in ocean monuments to light them. They can be retrieved with any silk touch tool, dropping 2-3 prismarine crystals (up to 5 with fortune) if not.

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.

Trivia

 * The animation shows a very faint 'rippling' effect. It is also currently the only solid block aside from prismarine itself with an animation in the default texture.

Gallery
Seelaterne Linterna del mar Lanterne aquatique Latarnia morska Морской фонарь 海晶灯