Ok, I'll admit it .... I'll be honest
I don't often close them. I know, I know,.... I should, but I'm at my boat every week. If I was away for a while I'd probably close them.But one thing I am NOT ignorant of is my boat. I know it very well. I check and look at my plumbing often, if there was something about a thru hull or valve or underwater hose or SS clamp that bothered me, then I would probably close it until I fixed what was bothering me.Yeah I know things can happen (and they do) so I am not advocating this for everyone. There are many designs and materials used on boats that would concern me more and make me close them(Cheap hose, old electrolysis ridden sea-cocks, etc)I was on a new motorboat yesterday that had the cockpit scuppers drain to an underwater thru-hull on the back of the boat that didn't even HAVE a way to close it. And the hose that was attached did not look like that really really heavy black exhaust hose, it was that hose that look like it goes on a vacuum cleaner. It looked open to any sort of loose object puncturing it and the boat would probably sink in 10 minutes. Not to mention that the bilgepump was so small I don't think it could drain a bath-tub (It was a "Sea Boss 255" walkaround)