For someone who is stranded on a tropical island, the three most essential things are water, food and shelter. Water is essential to keep the body hydrated and flushed free of toxins, while food helps sustain the body and provides energy. Shelter provides protection from the elements, such as wind, rain, sun and cold.
Water is the most important of the three items needed to survive as a person can live longer without food than without water. Shelter can be something as simple as staying beneath a grove of trees or beneath a rock ledge. In most cases, any water found will be undrinkable. If so, it is best to try to fashion something to turn seawater into distilled water. If a pot can be fashioned, water can be boiled and then cooled and be used as drinking water. Collected rainwater works in a pinch, too.
If stranded on a desert island, there may be tropical fruits around. Shellfish like clams and crabs may also be available. For those foods that are unrecognizable, it is best to test the food before eating, such as by rubbing the food against a part of the skin to see if a reaction develops. Then, taking a tiny bite and waiting to see if any stomach or other distress develops is a good idea, too.
Unless the island has caves other natural forms of shelter, making shelter will be necessary. Utilize any usable debris to form some kind of bed to keep the body off the ground to help keep away any reptiles or insects.