[Rich]

Are you two referring to different data?

When the law says 6 mph, isn't it speaking of the speed over water, not speed over ground?

Since boats operate in water I would think that headway speed has no reference to speed over ground, because current could in fact affect the speed as measured over ground.

So I would think a radar gun is more useful to measure speed over water than a GPS which without knowledge of the current measures speed over ground.

I have a sport type radar gun if you want to borrow it for your test.