Is there a way to partially submerge a Green Screen object/person onto/into UE 3D digital water with accurate water line and depth?

I am shooting objects on green screen, floating down a river. Using Composure I’m able to take the live feed from stage, and pre-composite in UE. However, I can only seem to create a rough mask for the water line. The camera is moving, and the object is moving, so a simple locked off mask doesn’t work. Does anyone have any ideas?