If i were on a deserted island i would bring a knife or tool, a water purifier, and something to start a fire with. the water purifier will make it so you can drink the salt water without it being too salty or gross. the knife/tool is a given, it can help you hunt for food, and do other things like cut wood for fires to keep you warm and to cook on. the lighter or matches will obviously help you start a fire to do things mentioned above.
