Having sex in any body of water actually poses a higher risk of infection to both partners but more so to the woman. Penetration allows other bacteria from the water to enter the vagina as well and can cause infection or irritation to the sensitive skin and membranes.
Water is not as lubricating as natural body fluids and can cause friction between the partners resulting in discomfort and irritation.
If water is forced into the vagina it can cause major infections and result in other problems for the woman as well.
If you have sex in water (pool, hot tub, etc) try to have the sex OUT of the water, as in, don't allow the genitals to be under water during penetration.
You have to be careful of irritation for the most part. Otherwise there's not much affect. The water on the person's body may cause him or her to be more or less aroused, it depends on the people.
Hope this helps, enjoy!
Between my gyno and spending time in the women's locker room for 24 years I've picked up a bit of knowledge ; )