The private pilot was conducting a personal flight over water in his light sport amphibious airplane. Recorded flight data indicated that the pilot performed a series of maneuvers near the shoreline at low altitudes. During the final maneuver, the airplane pitched up from about 210 ft, the throttle was moved almost to the idle position, the load factor increased to about 2 Gs, and the angle of attack increased steadily to 15° as the airplane reached 358 ft and banked more than 50° to the right. Immediately afterward, the airplane descended nose low and wings level and impacted the water. Multiple witnesses observed the airplane maneuvering between 5 and 300 ft over vessels in the water and close to houses along the shoreline. The recovered flight data indicated no exceedances of design maneuvering limits during the accident sequence. Even though the data indicated that the airplane might have experienced a momentary aerodynamic stall, the lack of reliable end-of-flight data precluded this determination.  Postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operations. The airplane performance study for this accident and the engine control unit data showed that the reduction in engine power before impact was a result of the pilot moving the throttle lever. Although bird feathers were found within the wreckage, no avian DNA was found, and there was no damage that could be attributed to a bird impact in the areas where the feathers were found.The airplane's flight track and orientation at impact were consistent with a loss of control. The Icon Aircraft A5 pilot operating handbook stated that the emergency procedure for a loss of control was to pull the handle for the complete aircraft parachute (CAP) system. As part of the After Cockpit Entry checklist, the pilot was to remove and stow the CAP system safety pin in case the system needed to be activated. Because the pilot did not remove the safety pin from the handle before flight, he would not have been able to deploy the parachute system to arrest the descent before impact. Although the airplane's altitude and attitude were not consistent with optimal CAP system actuation, the handbook indicated no restrictions on the use of the CAP system.   Page 2 of 15 ANC18FA007Postaccident toxicology testing indicated that the pilot had used multiple psychoactive substances before the accident, including amphetamine, zolpidem, morphine, fluoxetine, and baclofen. Although the effects of these five substances have not been studied in combination, each of the substances is impairing or potentially impairing by itself. Some of these substances might undergo postmortem redistribution, and the measured levels might therefore not represent the antemortem levels. However, all of the substances were at or above the levels that affect the central nervous system functions, including judgment, executive functioning, alertness, attention, and psychomotor skills. The substances that were simultaneously present in the pilot's system at the time of the accident most likely resulted in significant impairment that contributed to the pilot's decision to perform high-risk maneuvers. Further, the pilot's actions were consistent with sensation-seeking and a willingness to take risks. The pilot's risk-taking behavior was also demonstrated by his statement on social media (7 days before the accident) that flying the Icon A5 over water was "like flying a fighter jet" and his documented flight under a local bridge (12 days before the accident).In summary, the circumstances of the accident were consistent with the pilot's failure to maintain control of the airplane while performing aggressive maneuvers at a low altitude and a nose-high attitude, which resulted in the airplane's low energy state. The pilot's decision-making ability and psychomotor skills were significantly impaired by the combination of substances that were present in his system at the time of the accident.