I think your best protection is a pressure wash of the underside after beach use. Undercoat can protect from rock chips to some degree but not so good for rust protection. Sometimes undercoat (and bedliner) can get moisture underneath and hide rust.

Also don't forget to air down your tires... last time I ran on a beach 12 PSI was about right. With higher pressure I did not move. An air tank to increase pressure for the ride home helps.