Emergency Plus app (see "Emergency app that could save your life one day The Australian, February 24, 2015
A useful gadget I purchased recently is a keyring multitool. This is one piece of stainless steel about the size of a key, which has slots and knobs on it to act as a screwdriver, spanner and bottle-opener. More relevant for an emergency is the slot for cutting cord and webbing (such as seat-belts and backpack straps). The tool does not have an exposed blade, so it is safer to carry in your pocket and use under extreme conditions.
ps: If he is going to make a habit of falling into deep water while wearing a backpack, then David might want to invest in a Automatic Inflatable Life Jacket and water activated distress beacon. ;-)