Wouldn't that be rather dangerous? If the cooling system failed and the engineers were unable to get it up in time, then it could result in rather unpleasant consequences for people downstream.

But yes -- transparent dams would look awesome! :D

Pykrete floats.

Ice-rock (silt/sand/gravel) composites are a thing, so a pycrete composite might be made to sink. But that's still lighter than rock, so gravity dam overturning moment means the dam would need to be bigger.