Stuck thru hull valve
I assume that the valve is ball type; on my 1990 Legend 35.5 ball valves are used. You might try using a light oil or penetrating oil from both inside and out. Let the oil sit for a day then try to "work" the valve free. I was able to free up two such valves using the above method. Be certain to open and close the valves frequently to keep them free. A good way to do that, in addition to safety considerations, is to close all valves when leaving the boat and then opening them upon your return. Such a practice will keep them working properly.If the valve is a "gate" type valve, remove it and replace with a seacock or ball valve. Do the same with any other gate valves on your boat.PS