If you seal the wood good enough you should have no problems hosing it off after a day of fishing so long as you're not letting the water pool up on it for weeks at a clip. Use any good exterior paint and primer paying particular attention to the edges(I prefer oil based but for plywood latex would work fine, it holds up for years on houses). You could seal it with epoxy first to make it really waterproof if you wanted.
As for the sand, almost anything would work(mason's sand, play sand, heck you could even get a bucket of it from the beach). Just sprinkle it lightly and evenly(you only want a little bit just to give it some grip, you're not looking to make sandpaper) over a wet coat of paint and give it another light coat once it's dry to lock the sand in place and to "soften it up" a bit.