Funny. I wasn't planning on using the hand held for anything other than the initial calibration. I'll stick it in the water and compare it with the depth finder on the boat. And either recalibrate it or just note how far off it is. A line with a rock on the end and a measuring tape will do the same. But I have the hand held so why not bring it? I guess my point was, in answer to the newbie question was that you should not trust the depth finder on the boat. A couple of years ago we had a Sunsail boat. Didn't notice until we had left the dock that the depth finder was reading 100 feet. That's a pretty important (most important?) instrument and it needs to be useful.