There is a ledge at the south entrance to the great harbor on Mount Desert Island that is marked by a day mark. A few years ago the daymark broke off during the winter and didn't get replaced for almost a year. Even though I know where that ledge is, it was a bit disconcerting, particularly on calm days at high tide when the ledge was invisible. It was a relief when the new daymark was finally installed.

GPS enabled electronic charts were handy for passing near that ledge. It is nice that you can now download all the NOAA charts for free and run free or very cheap chart plotting software on a computer with a GPS puck plugged into it. On my powerboat my laptop with a GPS puck plugged into a USB port and running free charting software using free charts is my primary navigation system (I do have a chartkit too).