i believe that thing in the picture might just be a ruin, it may have been intended to be underwater almost completely.

It's the same ruin you can find in the mountains but in the swamp. Since swamp is flooded so is the ruin.
It's not a dungeon or anything.

Ah. OK. Thanks guys.

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.

I rolled by a swamp in my karve one time and sailed right over one of these (wells I call them) that had a waiting drauger and a lit green torch at the bottom. underneath my boat. totally submerged. this game some times

@bacon: A cautionary tale. Will take care to avoid that. Thanks.

@UncleAcidic: Even entirely submerged! This game is full of surprises. My surprise in that instance was the Abomination that was lurking next to it.

Thanks for the replies guys.