Bob,
I well recall your experience. It caused me to examine my sink hoses--and all hoses. Very educational story.
But I was wondering about P. Gazabera's case:
My friend took the boat out sailing in the Puget Sound up to Deception Pass with some visitors and while going to windward on a starboard tack he looked below to find water above the floorboards! Being the even-keeled fellow he is, he simply put the boat on a port tack and went below to find the leak. It turns out, the PO had done some work on the head sink and never properly reattached the drain. When the boat was heeled over water just came right on in. Luckily the old gate valve managed to close and no more water came in.
After discovering this, we realized that the rather bouncy windy marina the boat is moored in would cause the boat to bounce back and forth so much that water would come in through the sink drain when the winds piped up at the marina.
Lesson learned - close seacocks when not in use!
To me, the lesson is hoses, not seacocks.
I don't mean to beat this to death, and my opinion may be eccentric, but I invite sailors to consider the actual value in closing seacocks when "boat not in use."
There's obviously nothing wrong with it. I don't happen to do it, but most probably do.
However, if there is any danger of an open seacock causing a problem, at any time, under any circumstance, then I consider the boat unsafe.