Periwinkle,
Speaking from experience, I dumped my boat on the highway ten years ago and the problem was directly caused by a weight distributing hitch. The place that installed it didn't do the job right and my surge brakes locked and jacknifed the trailer. You can use weight distributing hitches but they have to be installed properly so that when you apply your brakes the coupler can slide forward as designed while your master cylinder slides backward on the rods its mounted on. This is how the braking pressure is regulated so you don't get lockups. Personally I would never use one on a boat trailer again. I would follow previous advice and adjust your boat back as needed to get the proper weight adjustment. If you have torsion axles you need to make sure the trailer is level when towing or you will be carrying all your weight on one axle.