Well, first off, this is definitely NOT a "no lose situation." If you end up, for whatever reason, not being able to use the boat as much as you expect, then you lose. If you aren't careful to read all of the fine print, and make sure you understand all of the conditions and costs, you could lose that way.
Other than that, though, this is a simple equation. How much--realistically--will you use the boat? How much would it cost you to rent a boat for that amount of time? If the answer is "more" then the timeshare is a good deal. If the answer is "less" then it is not a good deal. But you have to be realistic. The problem that most people have is that they say to themselves, "Oh goody, goody! I get to use the boat three times a month!"
The first month they use it three times. And the second month. Then the third month they only use it twice. Fourth month, once. Fifth month, not at all. Sixth month they're back to twice. See where this is going? If they had been realistic about how often they were truly going to sail, they'd have seen that renting
I'm not trying to burst your bubble, and I'm not trying to say that you won't use it as much as you think. I'm just saying that you have to be brutally honest when you are thinking about how much you are REALLY going to use it.
Good luck, whatever you decide. (Though, from the sounds of it, you have already decided.)