Ken, up here on great sacandaga lake because of the hydro dam and the need to supply water to the hudson river the lake can fluctuate quite a bit.
Many people have docks with wheels and also flotation so they can either float it or pull it in a little to keep it in contact with the bottom, then in the fall pull it out of the water.
http://www.greatnortherndocks.com/Docks/RollerDocks.htm