Seems to me, the worst case scenario is having to fix/replace something, but being unable to get to the bad bits w/o destroying the hull, wtc, etc. I've seen modeler boaters glue-in RC items (such as servos, with epoxy, yikes!), which become a royal pain to replace when they go bad, sigh.
Why has the sub never been wet? That might tell you something important about it's construction; for instance, maybe it developed a crack that is impossible to fix w/o destroying the looks.
Examine what you are attempting to achieve with this purchase before plunking down the money. That is, what will this "pig in a poke" be doing for you that you could not do better yourself?
This boat may perfectly fit your goals; since I don't know what they are, I can't comment on that

It's certainly tempting, to me, to obtain a boat that is mostly built and I have to just fiddle a bit with it....if you are a beginner submariner, though, fiddling may prove very frustrating - do you have a nearby sub club whose members could give you assistance?
Good luck, whatever your decision. You'll certainly learn lots, good and maybe bad, but that's all part of the game, I guess.