Salt water and sand...you couldn't find anything worse for wearing out parts on your bike and creating more rust than I'd care to think about. Any pressurized water for washing isn't a good thing either. Just using fresh water via a hose to try and flush as much sand/salt water off of the bike will better serve you. If you have an air compressor handy, a quick "dusting" of the chain should help, but don't blow any air towards wheel bearings or the bottom bracket. then a wipe of the chain and quick re-lubing will help a bad situation until the next drenching. If there are other local riders in the neighborhood, ask them their "secrets" for making their bicycles safe against the seawater and sand. They may have a few tricks up their sleeves.