A padlock of brass is really the only thing that'll survive in a marine environment without undue care. I have, on occasion, used hardened steel locks. A couple tips here. Lubricate it thoroughly inside the cylinder and then shackle openings with BreakfreeCLP, then load all openings with Bombardier 'Triple Guard' grease, a waterproof synthetic grease.
Then, place a small piece of leather, like 3" wide by 5" tall over where the lock goes. In humidty or other weather, including the saltwater environment air, all the moisture will go into the leather and not into the lock. This is an old trick my grandfather taught me and it works ... he founded the 2nd oldest locksmith company in the US.