Due to the routing of the hoses and EPA regs, it can be difficult to fully fill the tank. The best way seems to be getting the nose of the boat as low as possible. If you are on the trailer, try to find gas pumps where you are on a downhill slope if possible, the more the better.
If in the water, put everyone in the bow of your boat, to drop it down. It has also been discovered that weighing down the starboard side of the boat, while filling it on the water, seems to allow more fuel in the tank. Put some passengers on the starboard side, and/or bow while filling the tank! You can get it close to 100%!