Drainman automatic wave powered pump.
These really do work!
I own an open 13' dory (which is about the best rain catcher you can find!) which lives on a half tide mooring - much to my surprise the Drainman was able to keep the boat dry. I found best results were achieved by keeping the distance between boat and mooring buoy as short as possible so that when the boat pitched it pumped effectively, but if moored against a solid quay or marina berth this doesn't matter, as the video's show. With a breaking strain of over a tonne there are no issues on strength so its very simple to install and use. Environmentally friendly, uses no power or 'muscle', and can pump up to 2500 liters a day using only boat motion and wave power!