At a hydroelectric power plant, water is directed at high speed against turbine blades on an axle that ...?

Question

...turn and electric generator. For maximum power generation, shoul de turbine blades be design so that the water is brought to a dead stop, or so that the water rebounds?

Answer 1

Dead stop. If it rebounds, it slows the following water, reducing spin.

Answer 2

In practice neither would work. The used water needs to move elsewhere or the entire system will stop working.
In a wind system, the maximum theoretical efficiency is approximately 59%, assuming the wind came to a complete stop. This is the Betz limit. In practice this does not happen and overall efficiency is much lower.