Here are two suggestions from TV station KYES on how to receive TV signals that reflect off of water:
"In coastal areas, or across bodies of sea water such as Cook Inlet, the antenna can receive a strong reflection from the sea as well as the direct signal. As the water level changes, the standing wave patterns change too. Mounting the antenna such that the roof screens it from the unwanted sea reflection but not the direct signal usually overcomes the problem. In extreme cases it may be necessary to use two antennas - one positioned for best results at high tide, the other for optimum reception at low tide."
With too many variables affecting solid reception, cable may be your better option.