this is the same structure that can spawn at ground level, but near the coast, "ground level" can be underwater.

by very careful in these structures that are partially submerged. this is the only time in the game i became truly stuck, where i got into the shallow water inside the structure and it was deep enough to swim in a 1x1 block section, which was basically invisible in the dark water. couldnt climb back out, couldnt jump, couldnt drown, couldnt damage the stone. i think i had to enter and exit the world multiple times and try to basically glitch/slide as i was loading in.