Either leave it alone then or put a plug in the thru hull from the outside. I'm getting confused here. Are you worried about the gate valve rupturing, with the bonnet popping off and flooding the boat or are you concerned that you've a thru-hull you cannot close if necessary? If the former, then you need to replace it or plug it, period. If the latter, and the rest of the piping between valve and sink is in good condition, you can add another valve anywhere, even under the sink.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.