Not able to get buoyancy to work

I have been trying for days to get buoyancy to work. I have used versions 4.26, 4.27 and 5.0 with no luck. I enabled required plugins(water and landmass) and I used the buoyancy example that comes with the engine. I also followed several different tutorials and with no luck. No matter what I do the cube just falls, no floating.
Please help, I would like to use this feature.