Coïtus à unda is intercourse in water: in the bathtub, while showering, in the swimming pool, the sea or in a Jacuzzi.
Wait with intercourse till the girl is excited and her vagina is wet. If you start too early, when her vagina is still dry, you are making the entry difficult en moving painful. Water does not work as a lubricant!
Having intercourse in water is only safe if you both know you don’t have a STD (for instance because you never had sex) and if the girl uses oral contraceptives, an intrauterine device (IUD) or something comparable. Condoms are not made for coïtus à unda.
If heavy petting in the bathtub leads to an ejaculation, you don’t have to fear pregnancy. Sperm won’t survive the water. So it’s safe to take a bath or shower together and let your lover come by using your hands.