The airline transport pilot, a pilot-rated passenger, and a second passenger arrived at the seaplane base midday and planned to depart later that afternoon. When they returned to the airplane to depart, the pilot spent time draining the left sponson tank of water and fuel and replacing a missing inspection cover on the left wing. Before departing, seaplane base staff warned the pilot that the water conditions had deteriorated and took the pilot out on a boat to observe the conditions firsthand. The observed wave height was estimated 1 1/2 to 2 ft and exceeded the maximum demonstrated wave height for the accident airplane, which was 18 inches. The pilot initially agreed that the water conditions were too poor; however, he later elected to depart with a tailwind despite the unsuitable water conditions and strong objections of seaplane base staff and other seaplane pilots. According to witnesses, the pilot demonstrated a strong resistance to the advice of those who he interacted with throughout the afternoon.The airplane was towed out of the harbor by boat.  The pilot indicated several times he was going to start the engine while still being towed but had to be told to wait.  The pilot started the engine as soon as the boat and tow line were clear, applied takeoff power within about 10 seconds of the engine starting, and immediately departed downwind. The airplane accelerated on the water for about 60 seconds before it began pitching up and down, rose steeply out of the water, then rolled over to the left. The left wing impacted the water and the airplane spun left and sank. The pilot-rated passenger was able to egress, while the pilot and the other passenger were pulled from the airplane by first responders who witnessed the accident and responded immediately. Video footage of the accident, and the wreckage examination all corroborated that the engine was performing normally and the wing flaps were retracted throughout the takeoff. The airplane flight manual (AFM) stated that the wing flaps are to be extended for all takeoffs and landings, and a checklist stating the same was posted in a visible location on the instrument panel. Although the pilot-rated passenger recalled the pilot verbalizing that the flaps were in the down position before takeoff, evidence indicates the flaps were never extended during the takeoff sequence. The pilot-rated passenger, who was a flight instructor and had provided the pilot seaplane instruction, also Page 2 of 12 CEN17FA287provided a statement contrary to the AFM that flaps did not have to be extended for takeoff. It could not be determined whether the pilot was taught this during his flight instruction and elected to take off with the flaps retracted, or if he overlooked this step from the checklist.  However, the circumstances of the accident are consistent with the pilot rushing to depart and failing to ensure the airplane was properly configured with flaps extended before taking off. Additionally, the pilot elected to take off with unfavorable water and wind conditions despite the advice from other pilots that he not do so. The pilot's decision to takeoff with unfavorable water conditions and a tailwind, combined with his failure to lower the flaps for takeoff, likely contributed to the airplane stalling as soon as it became airborne and resulted in a loss of control.