The pilot reported that the engine gradually lost power (as if someone was slowly pulling the power off) during a climb from takeoff at about 300 feet above the ground. A witness (pilot) on the ground initially observed the airplane in an attitude which resembled a landing flare. The airplane then pitched forward and descended vertically toward the ground. The pilot said that he may have unintentionally stalled the airplane subsequent to the loss of engine power. The airplane crashed in a small unnamed lake, flipped onto its back, and remained in this state for the next 48 hours. A post accident external and internal examination of the fuel system hardware and the powerplant assembly did not reveal failures or anomalies that would indicate an inoperative condition. Likewise, the source from which the plane was fueled was unremarkable. The pilot did not possess a single engine SEA rating and had not completed a biennial flight review or equivalent in 40 months.