hey guys,if u have it gps in then if u go over it even if u could not see it that would be ok because if u did not see fish,why stop.u can see pvc on unit anyway ,I have seen it many times with 2d,si and di as others have already mention,and as for air in bottle or pvs yes it picks it up,now about fish sonar picks up more than air blatter for on a larger fish one can see whole fish shape and tell which way he is turn even see his fin,old rumors are hard to get away from ,these units today and some of these guys can do much more than adveradge guy thinks,hats off to them, they sure have help me a lot