Ocean Monuments have been reworked in Vanillabean. While not strictly a new structure, this page is here so they can be explained in detail.
Note: This is a challenging structure! It's recommended you read the Strategy section below before looting an Ocean Monument.
Ocean Monuments have been updated an replace the default Minecraft Ocean Monument structure (see Other Changes).
Ocean Monuments are now more similar to Trial Chambers.
Trial Spawners can be found here that spawn either Drowneds, Guardians, or Sunken Drowneds. Guardian and Sunken Drowned Spawners are less likely to be found in the main room.
Inside the main room are a guaranteed three Trial Spawners, one of which will drop a Gate Key upon completion. This is used to open one of the first three Trial Gates and enter the room beyond. Each of these three Trial Gates leads to a different pathway and a new series of challenges, finishing with an Elder Guardian fight. The Elder Guardian will drop another Gate Key which can then be used to go down a different pathway from the main room.
There are also a few hidden areas in the Ocean Monument, containing mostly Wet Sponges. You'll need to mine through Copper Grates to reach these, so it's recommended you defeat the three Elder Guardians first in order to lose your Mining Fatigue. Each of the three pathways through the Ocean Monument has one room with Wet Sponges in it.
Trial Spawners:
Main Trial Spawners
1 Gate Key
Other Trial Spawners (1 of these is dropped)
2-4 Bottle o' Enchanting
2-4 Wet Sponge
2-4 Prismarine Shard
2-4 Kelp
1 Potion of Water Breathing (8:00)
1 "saavane - Beach Vibe" Music Disc (only if Ominous) (see Custom Discs)
Other Loot:
1 Trident (found on top of the structure)
66 Wet Sponges (total)
7 Blocks of Raw Gold (total)
6 Blocks of Gold (total)
6 Blocks of Iron (total)
6 Blocks of Emerald (total)
1 Block of Diamond
The main difficulty in raiding an Ocean Monument is the fact it's underwater. You'll need to brew Potions of Water Breathing, or use Turtle Shellmets or a Tidal Tiara (which can also help when fighting in the structure).
If you already have a Trident, it's good to bring it, especially with the Impaling enchantment. Using a Tidal Tiara alongside an Impaling V Trident will allow you to kill Guardians in one hit.
Another useful enchantment, if you don't have a Trident, is Smite, which will deal more damage to Drowneds and Sunken Drowneds.
The Ocean Monument has many small nooks and crannies (especially in the main room) that can be used to rest, heal, and dodge Guardian attacks. Remember that time is important, your Water Breathing will always be running out.
If you have access to a Conduit, you can use it for a significant advantage. Just keep in mind it will be difficult to move while you're under the effects of Mining Fatigue.
Remember that Vanillabean updates Elder Guardians to give Mining Fatigue much more consistently. Don't bother bringing Milk Buckets or Ice Cream Buckets, as they won't be very useful.