Found a Underwater Ruin in the Deep Ocean, but none of it is actually touching the ground. It's just kind of floating there. Other ruined buildings in the immediate area spawned touching the ground. It seems like only the central ruin was effected.

seed: -8300275442758179407, x/y/z: 521/45/-454

Edit: Found another one in the same seed. This one even has an air block trapped under it. x/y/z: 814/38/-137, biome: deep lukewarm ocean