You've probably heard before that there is an underwater hatch on the Eastern part of the map. What I've come to tell you is that there is one on the west coast as well. I first read about it on Youtube, and then investigated it myself. It's a square hatch covered in sand with cables next to it. There is no tapping, but you will lose health once you get close to it.
Here's how to find it:
Go to the plane wreck off the coast of Paleto Bay. Then follow the pieces until you reach a cliff, then turn left. You should find a canyon with a crevasse. Go through this crevasse and keep on swimming straight. Even during the day, the four lights on each corner should be lit. If that's too hard to find, just go to the far northwestern part of the map.