Been there, done that
The trick is to not turn the thru hull itself. If you do, you break the sealant and risk leaks. If you look up into the thru-hull you'll notice two protrusions, they make wrenches that slip into the thru-hull and lock onto the protrusions. In my case I slipped the handle of a large adjustable wrench into the thru-hull, one large enough to lock on the protrusions, another wrench to keep the first one from turning, and a buddy to hold them while I climbed inside and spun the valve off. If the thru-hull does spin, it's probably best to completly remove it, clean up the surfaces and re-bed them with an appropriate bedding compound. That's how I do it.
