I have the Raymarine C80 fishfinder with radar and a transom mount ducer. Everything works great when I am in shallow water but as soon as I get to 500 feet I cannot read bottom or figure out how to set my FF up above 300 so that I can find fish. Will a thru hull transducer fix this problem for me??? Or does anyone have advice on how to set the head unit up to read above 300 feet??? I appreciate it greatly!!!