An environment that harbours bacteria
While freshwater and seawater both contain bacteria that our skin is able to tolerate, these bacteria are much less appreciated by our private parts. Indeed, having sex in the sea can increase the risk of developing a urinary tract infection.
Just like with having sex in a pool, the chances of getting fungal infections increase, as pool water destroys the natural bacterial flora in the vagina.
Other more technical problems
It is equally important to know thatseawater reduces natural lubrication, but also that of condoms. As a result, the condoms will be more likely to break and therefore increase your risk of catching an STD. Remember to use suitable protection or a silicone-based lubricant. Also, remember that salt and sand irritate the skin, so expect that effect to be multiplied for your private parts.
The rumour of the suction cup effect
The suction cup effect is probably the "danger" that all couples fear. The back and forth movement in the water creates a draught and the partners become 'stuck' in one other. This is an embarrassing moment, and the only solution is for a doctor to intervene.
Finally, we also have to point out the legal implications of this act. If you are naked in public areas, you risk up to one year in prison and a hefty fine - depending on which country you're in. To limit the risks, you could always just stay at home and try this out in your bathtub - just be careful not to slip...