Minecraft structure?

what kind of structure is that someone has seen before?

It should actually be underwater.

The Word generation may have made a mistake again - as so often

That could be an underwater ruin, but I'm not sure because I can't see exactly what it is. If you had taken a screenshot in Daytime, I would have been able to answer it accurately 😅

It looks like an underwater ruin to me.

That should (as the name suggests) own. Not be generated over water.