I have the 180 SP and it wasn't working right when I first put it in the water. It reflected huge differences in the depth, within seconds from 2' to 175'. I took it back to the dealer. They found it wasn't snapped in properly and it was missing a gel bag (if I recall correctly). Prob the same issue for you? I got a Garmin as back up.
You have to toggle through the Display options to find the water depth display. Press the MODE ("M") button repeatedly until you see DISPLAY. Then press the SET ("S") button. Now you can use the toggle switch to the right of the SET button to cycle through all the display options. One of them will be depth. When you find it, hit the SET button again to lock it as the current display mode.
