I've got a Garmin 500c fishfinder on the bow of a bay boat that I use strictly for depth readings. It reads perfectly in depths over 6 ft but won't display any data in shallower water except water temperature and since most of my fishing is in under 5 ft, the thing is useless. I've tried changing transducer frequencies, and changed just about every setting I can but still can't get it to display depths under 6 feet. Am I missing something?