pretty sure you can set density
looked on the forums and wheeled things have modifyable density
tested it ingame (echo(findClientByName("phflack").player.dataBlock.density); gives 0.7)
and looks like setting it to 5 drops me like a rock, and setting it to 0.1 launches me out of the water
you should be able to put that value into a playertype or edit it ingame as needed
edit: looks like water has a density of 1, so setting my density to 1 means I don't go up/down
and setting my density to 0 gave me a white screen, I don't think BL likes that (probably some divide by 0 while rendering? not sure)