The Ocean Monument is one of the most fun places to make your Minecraft base. Surrounded by the deep blue, the Ocean Monument is a fortress of strange stony fish and prismarine. However, it houses a great treasure of yellow- not the gold in the centre, but the sponges scattered around the monument. These we can use to our advantage.
In my newest world, I didn't even need iron tools to clean out the monument. Here was my strategy:
Needed materials:
-dirt and saplings to make a platform for trees above the monument
-cobblestone for furnaces and tools, as well as a bit of fuel.
-sand
-patience
-PEACEFUL MODE (I usually always play on peaceful mode;Bedrock is much more unforgiving when it comes to its difficulty.)
Phase 1. First, I swim into the front and begin to use sand to make breathing pockets for myself. Then, I plug up the front hole, set up the furnaces, and begin to slowly use the sand to clean out just enough to give me some breathing room as we look for a few sponges. Once they're located, I try not to drown as I use my stone hoe to collect the sponges and leave. Dry the sponges in the furnace, and proceed to phase 2.
Phase 2 is SpongeMania. Begin learning how to efficiently clear out rooms with sponges, and keep pushing through. If you find the treasure room or any of the wing rooms, block them up with sand for now. Don't be afraid to knock holes in walls, as long as you leave the sea lanterns alone. In fact, DO knock holes in walls. It'll make the place easier to navigate when it's fully dry. Make sure you're grabbing kelp at this point, and set up more furnaces to dry more sponges and dry more kelp. Craft the kelp into dried kelp blocks; they smelt 20 items each. Continue to expand; dry every room.
Phase 3 is when we conquer the treasure room. It's pretty simple, actually, just use the outlines on the top and section off floor to ceiling underneath the prismarine that sticks out, then use your sponges. It's more tedious than hard at this part, but that's nothing yet, becasue now we do
Phase 4: Wrapping up. Use the same sectioning off technique for the rest of the monument, which is a LOT of open space. This will test your patience, but once it's done, it's done.
Your monument is now cleaned out. What next?
This is mostly up to you. I like to enclose the little arches in the front with glass, and then modify the area in front of it by placing sponges to make it become a waterfall that doesn't flow in.
A tip on this is that using the sponges makes the water source blocks into flowing water blocks, which means they don't wanna expand outwards. If your monument is sunk into the floor, then just make a 1 block gap at the bottom so it has blocks to flow into.
I knock out a lot of the walls on the bottom and second floors to make it all pretty nice and open, and then completely collapse the third floor and make the fourth into my own suite. I'm also working on a nether portal system to directly connect other ocean monuments, artificially expanding the base via wormholes.
Happy base hunting!