Recreating The Last Scene From Titanic with a Green Screen Problem

Hey guys whats up. My church resently asked me to do some VFX for our church ministry and they want to do a parody of the last scene of Titanic when the girl is on a piece of wood and jack is in the water dieing.
Now I was thinking best way to go is a green screen but Im concerned about a few things.

1. jack hast to be lower than the girl seeing he is in the water
2. However, the girl actor still needs to be laying down
3. how to i make it look convenceing jack is in the water

I have seen stuff done when it comes to water will green foam balls or blocks and you just put your actor in all this but I dont have the time or the money to do this
any ideas?
Extra info:
10 x 20 Large Chromakey Chroma Key Green Screen
2 large white lights