After takeoff the airline transport pilot and pilot-rated passenger of the amphibious airplane performed landings at an airport and on a river, performed airwork, then proceeded west, where when near a river, both looked for wires and poles but neither saw any. The pilot landed uneventfully on the river and while on the step he initiated takeoff. Although the pilot was looking and did not see any wires or poles associated with wires that spanned the river, the pilot-rated passenger spotted unmarked powerlines spanning the river immediately before impact with them during the initial climb and called them out to the pilot. The airplane collided with the wire(s), and the airplane pitched down and impacted the river resulting in substantial damage to the right wing, right wing lift strut, and aft empennage. The pilot stated there were no preimpact failures or malfunctions of the airplane that would have precluded normal operation of the airplane.