I have seen exactly what you are looking for. Many years ago we did a seminar at Mameo Beach (Pigeon Lake) in Alberta, southwest of Edmonton. All the catamaran sailors who had summer cottages at the lake kept their boats on these skeleton contraptions made of pipe that were out in the water. Problem is, I can't remember how they worked. The boat just went into it or onto it somehow and they had some way of raising the boat up so it was suspended above the water. I can't even remember whether the boat was on a platform that raised up and down or whether it was on slings or what. I think maybe there was a big wheel they turned to do the raising and lowering. I know this is not much help, but at least you know there is such a thing. Maybe it was a local invention unique to that lake. Guess you'll have to take a drive over to Mameo to check it out.

I see Al beat me to it. Since he is from Alberta, he is probably talking about the same thing I am.