Maybe a collision mat?
It would be kind of expensive just to change a thru hull valve, but you might think about getting a collision mat. I paid about $150 at WM a few years ago for one. It's a triangle of plastic about 6-7 ft on each side with nylon lines in each corner. It's supposed to allow you to slide it along the underside of the hull on the outside to cover a hole that may occur in the hull if you collide with something. (The traditional name is "fothering".) The pressure of the water on the plastic causes the mat to be partially sucked into the hole, plugging it (or at least dramatically reducing the inflow to a trickle). Might allow you time to replace the valve if you don't have to make a run to WM in the middle of the job. Then afterwards, you've got a valuable piece of emergency gear aboard. Even at $150 (or whatever they are now), it's cheaper than a haul.Good luck!