In order to prevent air pockets, water generates around the monument as a part of the structure. It is the largest structure that does not contain a loot chest. They can be located with ocean explorer maps, which are bought from cartographer villagers with emeralds.

What loot does an ocean monument have?

The Treasure Chamber of each Ocean Monument contains 8 gold blocks, hidden behind Dark Prismarine. Prismarine itself is only found in Ocean Monuments, so if you want to use it in your own buildings, you have to take it from here. Sea Lanterns are useful light sources for underwater structures.

Is there a chest in a monument?

There will be only one chest in every monument. It will spawn randomly anywhere in the monument.