I don't generally use fill flash at the beach, I prefer reflectors. Part of that is because for a long time I felt the over-fill look (which usually results in washed out models and/or dark backgrounds) looked weird. I've been re-thinking that, and planned to experiment with it more this year, partly because I'd like to see more detail in the background and to help the models keep from squinting from the reflector (something that we deal with, but sometimes results in outtakes I wish I didn't have). Another reason for not using it is the exposure problem - the need to keep the exposure right on the money, which is something you have to learn to do when you shoot transparencies (slight dig at neg & digital guys). Since the flash exposure is based on the flash-to-model distance, the trickey part is keeping that distance constant when the model is moving around in the surf. Some guys use a string to easily re-check the distance, but again, when you're all bouncing around the surf, it is tough. I think shooting with digital might add some flexibility if only in the ability to more closely monitor the exposure and ratio of natural light to flash. Here's a couple examples of a simple reflector on the beach and in the surf:
Two benefits of reflector: 1) You can accurately see what the fill light is doing, and 2) To keep her from squinting, you can't always have it aim directly into the model. Moving it off to the side enhances the curves of her body. Works for me.
That said, I have used flash at the beach, of course. I love to use it at sunset with a warming gel over the flash like here:
I have used flash in full-on sun. Believe it or not, this shot was a test. A model friend dropped by the studio one day when the weather was great, and I had wanted to try the flash fill (with a 10Y gel over the flash, because unfiltered flash is too blue), and Velvia film (which was fairly new) so I three a bikini at her and we went to the beach. A couple years later I was able to sell this to Budweiser. Not bad for a test.
All the flash shots were done with (ready?.......) a plain old Nikon SB-26 in the hot shoe, in manual mode, powered by a Quantum turbo. No umbrellas, diffusers, etc. BTW, one nice thing about fixed focal length lenses.... If you're doing this, and the flash is mounted on the camera, you can help to keep the flash-to-model constant by keeping the model's size in the viewfinder the same. Meaning that if you move closer or farther away, you will know because she changes size in the viewfinder. With a zoom lens, you can move all over the place and not realize you've changed the distance. Also, don't count on this working with auto-exposure. I don't trust them, but with a digital LCD screen, you could try.
The models, from top to bottom are:
Andy Pearlman Studio