I usually fly 1000-2000 AGL, depending upon the type of terrain below for a forced landing. If it's really poor, and the winds favour, I'll fly higher to give myself more choice. If I'm flying the flying boat over water, it's an area I know with no boaters, and it's suitably calm, I'll cruise along at 10-20 feet above the surface quite happily. You just have to keep a sharp lookout for sea gulls doing that!