I would think 50m long by half that wide would be absolute minimum for seamasters and skilled pilots. One of the things I love about flying off water is that the landing area is huge, so more is better. It dosn't have to be very deep at all. A foot or less would work fine. Smaller planes could land in less area. We have 3 docks, spaced at 50 ft intervals and I can do two consecutive touch and go's in the space between them with my capricorn. I am always suprised how far a plane can stay on step with a hot aproach though, esp when it's headed for something solid..... It would help if the beach was forgiving and not a cement wall.