The floatplane took-off from a coastal lake in strong gusting winds associated with passing squalls. As the aircraft approached the coast, and flew over the beach at low altitude, it descended until the left float struck the top of a wave. The aircraft nosed over into the ocean and began to sink. The pilot and passenger managed to evacuate the semi submerged aircraft without injury. The pilot subsequently reported that, after taking off toward the south into the strong wind, he had to turn left at low altitude to avoid a small hill adjacent to the shore. The aircraft had then probably encountered windshear on the downwind side of the hill, which resulted in the unintended loss of altitude and subsequent contact with the water.