Most standard Transducers that comes w/ fishfinders this days would only go from 100 - 150 mtrs and the screen just go blank. Like a friend of mine has one of those Lowrance x515c and it says it can go up to 762 mtrs max and always goes blank on 120 mtrs. I dont know if his set up was wrong but another friend of ours whos got the same sounder does the same thing.
Possibly pm one of the guys like gregl about it as he does bluewater fishing all the time.