Motion detection under water does not go far enough to be useful. Instead you have to rely on video motion detection, which is not energy efficient.
I am thinking of doing the same, but most cameras I have looked at cannot be used long term. The fish cams are only standard def, and I want something better. This means devising a underwater sealed housing, and running wires up for continuous streaming feeds.

I just spent a bunch of money trying to address my leaking pond, so that budget is gone.