I use a Garmin GPS Sounder, primarily as a knot meter on local waters, and also for navigation when I travel. Something reassuring about pointing the bow towards a distant navaid that is 20 miles over the horizon and tracking straight to it and with an ETA being updated every step of the way. I always have paper charts and check the compass, but the GPS navigation has been flawless. Not only that, if you are able to read the sounder function correctly, it will tell you not only the depth to bottom, but also some notion of the bottom content, which is handy when looking for a place to anchor. But Dave is also right that in shoaling waters, there may not be much advance warning you are about to hit bottom, but that is no different than any other system. With the sounder function, you can even set the depth at which an audible warning sounds, so even if you are tooling along half asleep, the audible warning will alert you to trouble. Mine is an older version. The newer ones include digital charts that depict depth contours, so you can verify where you are and have a double reference......chart and depth sounder, to give you some indication of the water depth beneath you and what is in front of you. Combined with a compass, the GPS is also handy to give you some idea of the amount of leeway you are making. The one I have also includes graphs of tide changes for local waters. Can't tell you how handy that is in tidal waters. The GPS has also proven time and again that the Horizon knotmeter that is mounted through the hull of my boat, with gauge on the aft cabin bulkhead, is useless. I have not pulled the cover off that in over two years. It, nor the older unit it replaced, have every worked correctly.