Oh nice job ! looks like you beat me to it, I finished adding my buoyancy code to your yesterday and I put together a few objects of various shapes and densities… here is a video showing it 
Also added a pirate ship for good measure! Check it out 
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Btw, did you figured out what was causing the performance hit? it all runs the same to me, the buoyancy barely has any impact on performance if you use few test points.