I would say it depends on how badly wrong it is. It sounds bad if it is leaking in several places already. I'm in the northwest, here in my town even if you closed it off, if you were to put the house on the market a home inspector would call it out and it would be an issue. Of course it would be an issue also if it was leaking. I do a lot of work for sellers based on home inspections. it also depends on how far you are underwater. An extra bathroom will add to the value, but how much is the question. I personally would lean towards shutting off the water and leaving it. The idea be to do as little as possible. Just make sure that it can't leak anymore. Then if in the future you decide to start doing some work on it you can, without having to undo a lot of what you did to shut it down.
[FONT="Franklin Gothic Medium"][SIZE="3"][B]Everyone is entitled to their own opinion, but not their own facts[/B][/SIZE][/FONT]