So as I was exploring in my world into new chunks, I was flying over an ocean, and I spotted what I thought was a normal guardian, assuming there was an ocean monument. But as I looked closer, it was actually an Elder Guardian that had spawned, but no ocean monument, and no other guardians around.

In my opinion, this would actually be really cool to find guardians and Elder Guardians in the open ocean, and not just in the monuments. But I'm guessing here it appears to be a bug. It's still cool though.