I carry a small Tupperware tub which is totally waterproof. This box contains what I call my emergency kit. Broken down, this is what it contains;
First Aid Kit
A first aid kit that contains everything needed to deal with minor first aid issues. Things like bandages, plasters, sutures, gauze, medical tape, a sling, a scissors, a razor blade, cotton wool and any other basics that come with a pre packed kit. I added water purification tablets. First aid kits are widely available from outdoor shops and may other locations.
This is a small plastic pouch that contains a few items that could come in very handy if the worst happened and I had to take shelter on an island for a night. It contains items like fire tinder, fire starter, an emergency whistle and signalling mirror, needle and thread, duct tape, a button compass, tin foil and various other odds and ends that could make the difference between a vaguely comfortable ordeal and a horrendous one. The kit I carry is the Pocket Survival Pak available from www.dougritter.com
I carry a pack of mini flares which come complete with a discharger. Safety flares should need no explanation.
Folds up small and traps valuable heat underneath it if needs be. This item costs a couple of Euros from good outdoors and adventure shops.
Very useful if I get stranded at night. I use a small Petzl head torch which keeps my hands free.