A couple of days parked in the rain and the old Islander was carrying more H20 than AVGAS.
The tip tanks had caps which sort of covered the opening but provided no sealing whatsoever. This coupled with the upper surface sloping towards the cap guaranteed at least 3 or 4 litres of water in the tanks.
Better still though was the collection of water in the overhead panels which would douse R3 and R4 with a good bucket load when takeoff pwr was applied.