To be honest I have been wondering myself how to do buoancy. the tricky part if when only part of a shape is submerged. Then the upward force depends on the submerged fraction of the total volume.
Force field will either apply the full force or none, depending on the center of the shape (I think…). So you will get an constant up-and-down oszilation.
One way would be to split up the total shape in subshapes/actors. Another would be to append a number of points (?) from which you estimate the submerged volume, and then apply buoyancy forces yourself each frame.
PhysX has a feature in preparation which will solve this and other features of fluids: fluid heighfields. So I guess the best is to wait until this feature is released by NVIDIA.
enn0x