I pull the knotmeter quite often. Usually it is just to show folks how to do it. Sometimes to clean it.
I agree with Faster; the knotmeter is better for sail trim, if you are into that sort of thing. The GPS uses average speeds over a number of waypoints, the knotmeter is more instantaneous.
In new designs, there is a flap inside the the tube, that prevents a huge amount of water from getting in. It is not prefect, but it does give you peace of mind.
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As I sail, I praise God, and care not. (Luke Foxe)