by Andy Schell, SAIL magazine
Since I was a kid and for my entire professional sailing career, I took for granted that depth sounders read the water depth. Like, the actual water depth, as measured from the waterline. Obviously this requires a bit of adjustment since the sensor is never at the actual waterline, but normally a few feet below it, usually amidships just forward of the keel.

The way to do this calibration, incidentally, is dockside or at anchor with a lead line. Set the depth calibration to +/-0, then measure the actual waterline depth with the lead line in the same general position fore-aft as your depth sensor. The difference in readings is your calibration adjustment. For example, if your uncalibrated depth reads 10 feet, but the actual water depth as measured by lead line is 13 feet, there’s your +3-foot calibration.

But get this—not everyone does it this way! – Full report