Noisy locations are problematical by nature. Adding skimpy costumes makes it harder.
One trick I have seen done is to have towels draped around the necks of the talent and hiding the lavs and transmitters in the towels. Not the best solution, but better than none. You can use lavs on girls in bikinis, it just requires an extensive amount of pre-planning and someone who really knows their way around audio and costuming. Extremely tiny lavs like the CountryMan B-6 can be hidden in the hair, in baseball caps, in a necklace, even sunglasses and, of course, in the cleavage of the bikini top. The "secret" is to run the wire down the side of the talent that is not seen by the camera, and to change the position of the mic and transmitter with every shot/angle change. If they are seen from all angles in the same shot you have no chance, however.
If you have access to a recorder/mixer system that records multiple channels (four or more) you can use plant mics in addition to the boom (in a beach umbrella, a beach chair, a towel laying on the ground, behind a sand castle, etc.). And although it runs contrary to established procedure you can boom from underneath; less problems with shadows and the noise of the surf is mitigated a little bit as the mic is aimed at the sky, not downward towards the surf, and the sound is not bouncing up off of the sand.
Another thing to keep in mind is the humidity and the sand. You can put a non-lubricated condom over the mics to keep moisture and grit out of them.
Consider planning on getting dialog wilds while on location but away from the surf. The talent is still in character, etc.
Of course, you could just fake the beach by having them in a sandy area far from the surf and adding the surf sounds in audio post. You don't see the water, however...
Good luck!