Note that, to simulate water, you can't just set the world's gravity negative:
The rate a submerged object accelerates upwards depends on the mass of the object. Consider two boxes of the same size, one with a vacuum and another full of Styrofoam. Both float and both displace the same amount of water, (and therefore get the same upthrust). However, the one with the vacuum weighs less, and so the gravitational force on it is less. This means that the resultant vector (pointing up) of the Styrofoam box is *smaller* than that of the vacuum box. With a force on the vacuum box greater than that on the Styrofoam box, the vacuum box should accelerate faster because Force and Acceleration are directly proportional (F=M*A). But wait! Look at the M! The vacuum box weighs far less than the Styrofoam block *and* it masses less. Thus, the vacuum box's acceleration is even faster. Obviously a bit more complicated than anti-gravity... Ian
