My main water sport takes me out into saltwater with a wetsuit. In the past I have used a plain metal (no chips, no electronics) key on a rope necklace to unlock the door. I keep my main ignition keys hidden inside the vehicle.
Yes. Search for "replacement key ebay" here on the forum for the exact key # (I think it's MB-17). Then order it off ebay and take it to your favorite key cutter. Have fun.
I carry a plain metal key in my wallet. Some locksmiths have the blanks or you can order them. The key is a two sided cut, so it is a little more to cut it, but easy to do. MB-17 is the blank to search for - several threads as T2T indicates.