I just reread your post and it sounds like you might not have any key at all. If so, that is more of a problem.
One possibility is to replace the entire lock set as Capt Dan indicated. Here is the manufacturer's link for the lockset.
http://staging.southco.com/class/me-off ... -7409.html From it you should be able to figure out which lockset is a replacement for yours. if you download the literature, it has a ton of information. With some Google work, you will likely find someone who sells them as I think that Southco is wholesale only. A call to Southco might also get you the name of a retail seller that could order the lockset for you.
The other, as you now now, is that there are really only a small number of key variations. There are probably 50 of us or more on the site whose keys would work in your lock. The problem is that we don't know who we are. Since it stopped pouring here, I went out to the boat to see if there was any code visible on the lock set and I could not find one. One solution would be to go to a C-Brat gathering, like the upcoming one in North Carolina and put a sign on your boat. Cold six pack for the first person whose key works in your door and who will give you a spare. Highly likely that such an effort will provide a solution. Also, recognize that the lock is used on a lot of other brands including Sea Rays. Tell all your friends your story and see what their boat keys look like.
Finally, I don't know whether a good locksmith can disassemble a lock and make a key from scratch. If so, I suspect the cost will likely be near the cost of a new lockset.
Hope that helps. Not having a key would be a real issue.
Jim