This might be sacralidge, but personally when I'm at the beach I usually leave the big gear in the room safe.Sacrilege And no...
My recommendation is enjoy the beach! Get a waterproof camera, they are awesome fun and since you're on the beach in the sun low light performance isn't too important....this is not sacrilege but excellent advice. The 450D is going to have a hard time withstanding the sand, the 5D3 might fare better but swapping lenses is out and you don't want to be attached to that expensive piece of kit when you want to be chasing chicks into the sea. Or something. Check out the many great waterproof camera's (I'm particularly impressed by the *cough* Nikon AW1) that you can get for peanuts these days, and go splash around.