Be buff, lol
Seriously though. Having a great physique and some nice, trim swim trunks is the way to go. You also can't go wrong with a minimalistic sport watch and some dog tags. If you don't have the greatest body, and you won't be in the water, a light-colored linen or chambray button up with rolled sleeves works really well. Pair that with some cotton or linen shorts and a pair of boat shoes or driving mocs (go barefoot ON the beach of course), and you've got yourself a nice preppy look for the beach.
If you want more of an edgy look, try board shorts with a sleeveless t-shirt or rashguard. Careful though. You need to be younger and in at least some kind of decent shape to pull this off.
Bottom line, for the beach, buff is better. Add a bit of scruff and some raybans, and you're good to go.