Like nudes, any sexual energy often has the effect of making the subject self-conscious and the pictures feel contrived. That said, swimwear is a world of its own because of the fun factor. The art here is to create a brother-sister type of environment and to have as much fun as possible. Often images that appear steaming and provocative on the published image were actually made between a lot of laughter and misbehavior. History also tends to play a roll. If a model is comfortable with you shooting them semi-naked in a studio then it's an easy transition when you find yourself with them outdoors and partially clad.