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It sounds to me that what you did was the right way to go. The lift being negative is not unexpected as the velocities at the hull surfaces due to its curvature would induce higher velocities - high velocity = low pressure.
Obviously, if the free surface problem was modeled there is a hydrodynamic lift component that assists in obtaining a planning condition but I would think the approximation might be too rough to capture that effect. The only componnet of the drag force would be the skin friction and possibly to some exent the pressure drag might be incorporated. However, the wave drag would not be accounted for in any way.
You should also check to see if turing on the pressure potential siwth has any influence - you do this by turning on gravity and in then in the initial conditions dialog the pressure potential switch should be checked by defualt but you should check it to be sure. I would make a run with this enabled and one without and see if it makes a significant difference. The switch will account for hte pressure differentials due to gravity to be accounted for. I am not sure off he top of my head how significant it would be in you case.
The other thing you could do is modify the wall you put in to account for the surface of the water and shape it to approximate the free surface deformations if you have any feel for those and see if it provides andything useful. You would be way out on a limb on this one but one nevers knows.