The two I have aboard currently are older models that actually screw into a metal deckplate. In heavy weather, or if I anticipate going out in really bad conditions, I can unscrew the vent and plug the hole with a screw-in bronze deckplate. This is about as waterproof as you can get.
That said, I've yet to get water into the boat through them, and I've had waves wash over the cabintop.
I am installing a less robust model for the anchor locker to help keep the anchor drier.
The new lower end models have a sliding sleeve that can close off the vent and prevent water intrusion in 99% of the cases... and seem to work quite well, but I've no personal experience with them other than seeing them on friends' boats.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.