I found myslf heading up the Niagara River yesterday after a dive on Lake Ontario. We made it up to just short of the Whirlpool Rapids before the water got pretty scary. The difference between GPS speed and speedo was about 10 - 12 mph...
Which of course meant we hauled ass coming back down...
With my nice new engine, we managed to hit 60.5 mph on the GPS, with 2 people, 2 dogs, and a load of dive gear!
I must confess, it was moderately terrifying... but damn fun!