I found this going out over the ocean one time. I could see lights at the ocean floor and wondering what they were, I drowned myself trying to see what it was. I got far enough down before I died that I could see an end portal inside. The coordinates below are a maybe because I had to go back to search for it, and I'm not sure if I found it again (it's kind of hard to see from the top of the water). We can take an eye of ender out there to try and get a better location. We'll need potions of breathing or an enchanted helmet to actually get down there.