These three items address the fundamental needs for survival: water, shelter, and the ability to create fire. While other items like a fishing rod or a first-aid kit could be helpful, these three offer the most immediate and versatile solutions for a deserted island scenario. You can survive for up to 2 weeks or so without food, but you'll only last 3-4 days without water. Use whatever resources you have to build a shelter to keep you out of the sun and any bad weather. Start a fire. Fire will allow you to purify water and cook any food you may be able to find.