Okay so this would've been somewhere from 2012-2014 and on Xbox 360. I don't know the seed because this was on my cousin's Xbox and not mine. So my cousin and I were playing and he was doing pretty well, I don't think he'd gone to the end (if the end even existed at that point I'm not sure) or anything like that. We were on a boat and were in a lake of some sort and I noticed something underwater and I wanted to explore it so he agreed. All I really remember is that the structure was massive and had large and intricate hallways. My cousin might've built it but I don't think he did because he didn't mention anything about it. Does anyone have any idea what this might be?
No, it definitely wasn't a stronghold. It had the blocks of one of the ocean monument but particularly one hallway that must have been at least 100 blocks long. I don't think the inside had water but it was definitely an underwater structure. Looking back on it, it's probably just something my cousin built and showed me and I'm not remembering it right.