Ocean Structures
Structures found in or near oceans, including monuments, shipwrecks, and buried treasure.
Single chest buried under sand or gravel
Large underwater prismarine structure
Ruined stone or sandstone buildings
Wooden ship structure, partially destroyed
Which ocean structure is worth the dive
Four structures generate in or right next to the ocean, and the chests in two of them can hold the map to a third.
Ocean Monument
Three elder guardians live inside, and while they are alive they keep giving nearby players Mining Fatigue, which makes breaking through the walls painfully slow. The treasure chamber in the middle hides eight blocks of gold, some monuments have rooms with about 30 wet sponges on the ceiling, and each elder guardian has a one in five chance to drop the Tide armor trim template.
Ocean Ruins
Stone brick ruins in normal, cold and frozen oceans, sandstone ruins in warm and lukewarm ones, usually with drowned close by. About 30% generate as large ruins, and a large ruin often has a group of small ones around it. The chests can hold a buried treasure map, and brushing the suspicious gravel or sand turns up pottery sherds. Warm ruins are the only place to dig up a sniffer egg.
Shipwreck
Up to three chests, depending on how much of the ship survived: supplies in the bow, treasure in the upper part of the stern and a map chest below it. Find the map chest first, because buried treasure maps come from shipwrecks and ocean ruins. Some wrecks lie beached on the shore, so you can loot them without getting wet.
Buried Treasure
One chest, usually under beach sand and now and then on the sea floor, and every one holds a Heart of the Sea. Nothing else in the game gives you that item, and you need it for a conduit. In Java the chest always sits at position 9, 9 inside its chunk, so once the X on the map is close, press F3+G to show chunk borders and dig there.
Finding them
With cheats on, the /locate command finds all four: /locate structure minecraft:monument, minecraft:ocean_ruin_cold or minecraft:ocean_ruin_warm, minecraft:shipwreck or minecraft:shipwreck_beached, and minecraft:buried_treasure.
Without cheats, apprentice cartographers sell ocean explorer maps that lead to a monument. For buried treasure you follow the map from a shipwreck or a ruin, and the chunk position above tells you exactly where to dig once you reach the X.
Before you dive
A few Potions of Water Breathing are enough for a shipwreck or a ruin. A monument asks for more: Respiration and Aqua Affinity on a helmet, and milk to clear Mining Fatigue, although the elder guardians will put it back until they are dead. Keep your first Heart of the Sea for a conduit. Craft it with eight nautilus shells, place it inside a prismarine frame, and it gives Conduit Power to players in the water around it.