Tip 3.) when you do your boat briefing / handover, check if there is an offset on the echo-sounder. Our first year someone had put 4ft beneath the keel 'to be safe'...so it helpfully read zero all the way in, which is bugger-all use to anyone. We now ensure that it is calibrated correctly, without any offset.
Having an offset on the depth sounder is not necessarily wrong. So don't wipe it out without first checking the depth on the sounder vs. reality. On my boat the calibration offset is 4 ft on the instrument. Which will give me the true water depth on the display. I always bring a sewing measuring tape (i.e., vinyl) attached to a larger lead fishing weight. At the first opportunity I get the depth and compare it to the sounder and adjust accordingly. A depth sounder that is lying to you is absolutely useless and in fact worse than not having one at all. Imho.
