the "tweener" boat hoist designs that I have seen (and contemplated ever since I started considering purchasing a paddle/pedal boat) are a simple inclined ramp made of treated lumber. The ramp is part of the boat dock. The lower portion of the ramp in just under the surface of the water at a point beneath the front draft level of the watercraft, and the balance of the ramp continues up and out of the water. The ramp would be supported by floats or barrels so as to fluctuate with the static water level.