Ocean Monument

Water Dungeons, also known as Ocean Temples or Monuments, are naturally generated structures that will appear in Deep Ocean biomes, however they can sometimes be found intruding on nearby regular ocean biomes. They will appear in the new 1.8 snapshots

The dungeons will consist of types of Prismarine and will be lit by Sea Lantern blocks. The dungeons will consist of at least six rooms.

One of the types of room that can spawn is a room that has Wet Sponge covering the ceiling.

These dungeons are large and pyramid-like and contain many Prismarine block types. They are inhabited by Guardians. Inside the main entrance, there is a spire that goes up through the dungeon. At the top floor, there is an Elder Guardian that acts as a boss, though is not an official boss mob. .

In the protrusions of the dungeon (the parts of the building that go either side of the entrance), there are more Elder Guardians (one for each protrusion). The space they occupy is part of a giant room that makes up the bottom section of the dungeon. In the back middle of this room are the 8 Gold Blocks, encased in Dark Prismarine, the only treasure of the dungeon.

The Elder Guardian drops Prismarine Shards, raw fish, and a Wet Sponge. The new Guardian mob also spawns around the dungeon.

The ghostly image of an Elder Guardian and the infliction of Mining Fatigue III when approaching a water dungeon is caused by the Elder Guardians within the dungeon, not by the dungeon itself.

A good strategy for exploring the area in Survival is with invisibility and water breathing potions, paired with Depth Strider enchanted boots (only one piece of armor may be used in order to avoid Guardian attack.)

Trivia

 * The base area of the Water Dungeon is 58×58, quite a bit larger than Desert Temples (21×21) and Jungle Temples (15×12), and somewhat smaller than the average Stronghold.