The pilot reported that after departure he proceeded to a nearby lake in order to practice water takeoffs and landings. After landing, the water conditions became too rough to safely takeoff. The pilot elected to water taxi to a bay area near an island for smoother conditions. He stated that as he crossed near the southern tip of the island, the airplane struck a submerged object. He estimated the airplane's speed at the time as approximately 6 miles per hour. He steered the airplane toward deeper water; however, after about 3 minutes his feet were under water. He subsequently taxied into water about 3 feet deep and came to rest with the landing gear down. He noted that the airplane normally sat about 18 inches deep in the water. The depth finder did not alert him to the presence of the submerged object prior to the collision.