Close the thru-hull.
You've just illustrated why ALL thru-hulls should always be kept closed except when whatever they're connected to is actually in use. Unfortunately, that's not always the easiest thing to do, because seacocks definitely aren't "decor" items, and today's typical boat buyer thinks they detract from a boat's sleek "floating condo" appeal...so despite the fact that ABYC standards (which are totally voluntary) call for all seacocks to be readily accessible, boat builders hide 'em under settees and other places that make it very inconvenient to get to 'em. So they stay open all the time...because who's take everything stored in the settee out--or worse yet, remove the drawers under a berth or go out in the cockpit when it's raining and take everything out of a cockpit lazarette--and then put it all back, to open and close a seacock every time you need to wash your hands or use the head? There is a work-around: if you can't easily get to the seacock each time you use whatever is connected to it, install a shut-off valve that IS readily accessible...in sink drains, head intake lines, and head discharge lines if they go overboard), and keep those closed except when actually in use.
