I was swimming around the plane crash site underwater with a scubatank, and decided to swim a bit farther North-West to see how deep the guy could swim. I then went past some underwater arch structure that was farther away from the coast, and was very close to the depth line, and decided to still keep going. In that direction, on the floor of the ocean, found this strange cement slab with two pipes and four lights, one in each corner of it:
(Ignore the two vertical black dots, used those for another game)
Exact location on map:
(Sorry for the crap quality of my phone, I have no idea how to take screenshots or even videos on my friends XBox)
I could not get any closer to it than where I was in the screenshot without dying (being crushed from depth). I heard someone found something similar on the East side of the map, but they never gave specifics of where other than on the East. The description of it they gave was similar to what this looks like, and they also said they could not approach it too close without dying.
Honestly, I don't believe it's anything special, probably an oil pipe decoration or something, but I found it odd how it is so close to the depth barrier, and that it had four lights. I also could not get a good glimpse of whether there was a hatch or something like that, but I doubt it.