The commercial pilot was conducting an on-demand air taxi flight with three passengers on board the multiengine airplane. The majority of the pilot's multiengine experience was in a larger airplane with more powerful engines. He had about 20 hours of experience in the accident airplane make and model and had completed training in it about 9 days before the accident. The pilot stated that, after receiving clearance for takeoff from air traffic control, he performed the takeoff roll and rotated the airplane at 85 knots. The airplane climbed to about 100 ft above ground level (agl), but then did not continue to climb or accelerate. The airplane then yawed left, and the left engine rpm needle indicated less than the right engine rpm, but it remained in the green arc. Subsequently, the pilot turned the airplane left with the yaw to return to the airport, but the airplane was unable to maintain altitude and subsequently impacted water. The pilot further stated that, other than the rpm difference, there were no other preimpact mechanical malfunctions with the airplane.The front seat passenger reported that he was not a pilot but that he was familiar with airplanes. He stated that the airplane initially gained some altitude, but that it then yawed left. He then noticed that the pilot did not correct the yaw and let the airplane turn perpendicular to the wind, at which point he mentally questioned the pilot's competency and felt compelled to take the controls, but he did not touch anything. The airplane lost altitude and then flew under a tree line while remaining in a left banking turn. The pilot then moved the yoke abruptly right, and the airplane collided with the water. The front seat passenger added that the sound of the engines did not change during the flight and that he noted no smoke or fuel odor.Airport surveillance video confirmed that the airplane climbed to about 100 to 150 ft agl, which is out of ground effect, as it passed over the departure end of the runway. The airplane remained in a nose-up attitude as it then began to descend and, begin a shallow left turn; it then impacted the water.Examination of the airplane did not reveal any preimpact mechanical malfunctions that would have precluded normal operation, and the airplane was about 350 lbs below its maximum gross weight. Page 2 of 11 ERA17FA195Additionally, the airplane was equipped with constant-speed propellers, and a loss of left engine power would have been noticeable on the manifold pressure gauge and not the rpm gauge.The airplane's airspeed indicator revealed that mph were depicted by the larger numbers on the outer ring and that knots were depicted by the smaller numbers on the inner ring. Given the airspeed indicator's configuration, it is likely that the pilot rotated and then attempted to climb the airplane at 85 mph and not 85 knots, but either airspeed was significantly less than airplane's best rate-of-climb speed of 120 mph (102 knots). It is likely that the airplane climbed at the lower airspeed until out of ground effect, but then could not maintain a climb and began to descend back into ground effect. Further, the airplane was not equipped with counter-rotating engines to offset the left yaw at slower airspeeds, and the airplane began to yaw to the left. Rather than lower the nose, correct for the yaw, and continue straight into a 15-knot headwind to increase airspeed, the pilot allowed the airplane to continue to yaw left and exceeded the airplane's critical angle of attack, which resulted in an aerodynamic stall and subsequent descent into water.