When security is important to you, locking everything up becomes a necessity. Most pad locks will not survive for long in the marine environment and will need to be replaced due to corrosion issues.
I never gave the pad lock on my boat much thought, as it was there when I bought the boat and it is used everyday without issue. I saw an article a few days ago about "Searching for the perfect boat lock" and that's when it struck me how lucky I am to not worry about such things.
I don't know how old my lock is, but it was exposed to the elements for 3 years while the boat sat for sale, waiting for me to find her. There was no cover over the lock and it was exposed to rain, sun, winters, and summers. When I bought the boat, I turned the key and the lock opened up without a hitch; just as it has done everyday since.
I am not saying that this is the "best lock in the world", simply that this is a good lock that works day in and day out.
The lock is made by Abus, and is not readily available. When I had copies of its key made, the locksmith had to call around to see who had blanks for this key. I have seen these locks for sale from time to time at West Marine and small locksmith shops.
Abus does make "Marine Padlocks" that have all sorts of seals to keep moisture at bay, but this simple No 55/40 seems to work just fine. If you are having lock issues, consider an Abus lock.