The problem valve: About 4" above is blocked. There is about 6" on either side to work with, and as you can see the area behind the valve quickly tapers off to nothing. This valve is about 12 - 18" below static waterline. The ball valve in front of it is for the holding tank discharge.
I am concerned about having a thru-hull that can't be closed. I also can't winterize the system properly. The sink drain is 1 1/2" exhaust hose that appears to be in good condition, but then it tees off. This is a really tight area. It passes through a bulkhead about 6" after making a 90 degree turn, and then under some cabinetry and then to the sink. The other problem is not being able to winterize the system. I usually pour RV antifreeze into the sink. It does get cold enough to freeze sea water here!
Would life-caulk cure in time? It looks like I am going to be on the grid for about 12 hours. The next high tide isn't high enough to float off. Weather looks like chance of rain. Highs 45 low of 36 Shorepower is available so I could use a space heater if necessary at least inside the hull.