Good luck! Between my own experiences with salt water ferries (and their fishing boat substitute a couple of times) and wrenching in a shop only an hour away from the Atlantic I will say that rust and other oxidation is inevitably. The first best first step is avoidance. Next is frequent rinsing with fresh water and general cleaning. Third is relubing and waxing (or other protectent). While a tarp might trap moisture If the bikes are above deck and the seas are rough I'd rather ,minimize the drenching that they'll get. Andy.