Testing the Waters
This is easier than it sounds. Just load up the free ride track "Devil's Playground" and go to the south end of the map where the water is. Then find a spot where the water cuts inland. There just take a huge running start running parallel with the water then where the water cuts inland just pull back on the right analog stick and just hydroplane until you lose momentum and sink. If the trophy doesn't pop up, just keep trying or try a different place along the coast. You do NOT have to do this without sinking, just hydroplane at least 120 feet and then you can sink into the water.