I agree, with the logic. it makes perfect sense. you can only go hull speed. Agreed/
but in the real world the current, and winds especially in confined spaces, change or vector quickly and without warning.
So you are making 3knts against a current, and you are heading right into it , but then it shifts, (or the wind does), and pushes the bow to one side. you have about 2 seconds to correct or, the boat will be broad side, and then it will be trouble.
That river Sumner posted is protected, but it traverses unprotected bays, on the way out. if you were fighting to go up river with a gusty cross wind, you really need to be alert.
(late post)...
I think current is NOT constant in cuts, channels, and rivers.
Due to the bottom and change in widths
and if you punch it on a small prop, it cavitates more. so you have to apply throttle slower to get the bite and turn the boat.
in a larger bay, like tampa bay, or charlotte harbor its pretty consistent, until you get to boca grande. where it picks up, like a river...