The lever on the ball valve operates freely, but it doesn't feel like anything is moving, and the water flow to the strainer doesn't stop completely. Tightening the nut at the lever base doesn't do anything... because the nut turns but doesn't actually tighten.
It's a very cramped area, and difficult to see & work in. Since this is the raw water feed for a currently in-operative AC system on a recently purchased 10 year old Jeanneau, I would be satisfied with any fix that would stop the water at the thru hull-ball valve, that did not require a haul out, but that would keep the boat safe. She will be stored afloat this winter (with a bubbler) in the Chesapeake.
A word of caution about trying to do it in the water. If the thru hull is not bolted thru the hull, when you try to unthread the ball valve you may turn the whole thru hull, which of course will then leak :-(( Doesn't sound like you have a lot of room to get a wrench on the thru hull.

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