I hadn't thought of this other than in a general manner. We mark our charts with notes such as: "good trees" (to tie up to), "Wait traffic" (a good safe place to pull over when northbound to allow southbound traffic to get through critical areas), and we mark radio channels that facilities (bridges, locks, boat stores, commercial docks, etc, use). It makes life much easier when trying to memorize a couple of thousand miles of waterways.

Per your idea, I would get an overlay and mark your geocaches, water sources, camping spots, etc on it. Keep it separate from the map until you need the info, then lay 1 over the other. That provides a little better security for your information should the wrong people find one or the other.

Me, being my normal paranoid self!