I think the system was really not made to be used like lakes and I am almost sure someone tried this before and never made it to work properly and gave up. It would be less troubling implement buoyancy on another asset … there is even a guy who implemented a simple one which works nice, you could try this: 3D Projects - Elhoussine Mehnik
He has two projects implementing buoyancy, one for ships and one which is simple buoyancy. It worth check them out.