Exit velocity is the nozzle exit speed. The water exists the nozzle and hits the turbine.
With that said, turbine efficiency is not strictly based on resistance. Rather it is about extraction of energy. In a few designs it looks like resistance, others it is about redirection: https://www.energy.gov/eere/water/types-hydropower-turbines
I’m completely unfamiliar with any of this stuff so please keep this in mind…
If that’s the exit velocity, shouldn’t the turbine it’s spinning be geared to give a bit more resistance?
Or is that the velocity of the water if its path is unimpeded?