-> Use Editor to place a car (player) close to the sea.
-> "Preview"
-> Drive close to the sea while not driving not faster than app. 40-50 km/s or you're gonna be hurt or even die while ejecting
-> Eject while the car drives into the sea (Attention: It might go wrong sometimes, if you miss the right moment to eject, but for me most at a time it worked)
-> watch yourself hit the ground underwater, after a few seconds you'll be able to look around but not able to move. (As shown in picture)