The pilot reported that just prior to commencing descent from a cruising level of 1000 ft, he began to feel slightly ill. As the descent was commenced, he felt violently ill and dizzy. He opened the side window for some fresh air. The passengers stated that the pilot's hands then dropped from the controls and his head fell back in a faint. A passenger in the right control seat, who had some piloting experience, took control of the aircraft while another passenger seated behind the pilot tried to revive him. The flying passenger made plans to land the aircraft near the shore but after several minutes of unconsciousness, the pilot recovered sufficiently to land the floatplane. The pilot sought medical attention and was taken off flying duties until the incident was assessed. During this time he was ill for several days with a stomach upset, as were other members of his family. The medical investigation did not reveal any underlying problem. The medical opinion was that the pilot had suffered a vaso-vagal reaction in the presence of possible gastronenteritis.