SEAL's never have problems with sand jamming their H&K's. That's a bit why I selected mine.
I carried a few times on the beach in Charleston, SC, in my full coverage Action Direct chest pack/holster. My H&K was always wet with perspiration after hard exercise, and never the worse for wearing it.
As to cover, water is great cover, particularly at a distance and shallow angle. I'd be in the water in a flash.