And in case anyone is interested, I finally reached it and killed it. I mined down one of the ocean monument support legs until I was at the right level, then made myself a glass tunnel, much to the bemusement of the random guardians down there, and then made a door to stick my head out of and shoot. Took 40 arrows with a basic bow.
I will now put a sign up to mark the location. Could be a tourist attraction.