My guess is that lift = drag. If it produces lift, the drag will increase.
The more air that goes UNDER, rather than around or over, the more drag will be produced. Therefore try to shape the sides so that the air will split in such a way as to direct air to the sides and top.
The outlet at the back should probably be slightly larger than the inlet. Wind tunnels sometimes do this to prevent tunnel "buoyancy".
A movable surface at the back would be a good way of tuning the lift-drag balance on the water.
These are all just hunches - I have no actual experience with these hulls.