Recently I stumbled on one of those underwater monuments after spectacularly missing the mark on portal navigation. I tried to place a nether portal so it would connect to one I made in a distant village, but instead it deposited me deep underground next to a lava pool. When I started blasting away at some nearby dirt in search of ores, suddenly the ocean broke through the ceiling. I wasn't expecting that at this depth. After nearly drowning on the way up to the surface, I saw the lights of the monument in the water and the guardians swarming around it.
After a few trips to and from this new portal, I was sufficiently supplied for an underwater expedition, with Depth Strider boots and potions of water breathing, and I made my way into the monument interior by tunneling under the ocean floor and up through one of the pillars. I killed two elder guardians in my initial exploration of the interior, but navigation was still quite difficult in the darkness and wetness, especially with all those fishy guardians flitting around. So I decided to make the rooms breathable by draining the water until it was only one block high. In a fit of obsessive compulsion, I spent nine hours of my Saturday making every space in the monument breathable, killed the last elder guardian, and harvested all the sponges inside. Now I can rest easy.
I have no idea what to do with all these prismarine blocks though.
