Use the Vortex between El Quebrados and the abandoned airfield to take the small unique jump ramp that's on the back of the small shack it's parked next to. When you land in the water, the unique jump slow-motion cinema should keep going, with the camera fixed in one spot. Drive way out into the reservoir, and towards the dam, to the left of the camera's view. You'll disappear from view, but keep going. Eventually you'll run into the side of the dam and the cinema will end, leaving you under water and able to steer the Vortex like a submarine, without draining lung capacity or anything, as if you weren't underwater at all.