This is strictly guesswork, since I've never used any swim classes...
UpBuoyancy: float
How much the avatar buoys up while in the water. Imagining a flat surface slightly underneath the water as a "center", this would be how far above the plane the avatar will float up.
DownBuoyancy: float
Same thing as above, but downwards instead.
MaxUpwardVel: float
Maximum Upwards Velocity. How fast the avatar rises to the surface of the water.