You can only survive three days without water. Scavenge the beach for containers and set each one in the sand, at the bottom of a wide, shallow pit. Line the sides with palm leaves, arranged so that rain drips into the bottles. While you wait for rain, explore the island to look for streams.
Coconuts are a great food source. Wedge one on the ground, with the pointed end facing upwards. Find the largest rock you can lift and drop it onto the coconut to break the husk fibres apart. One survivor of the 2004 tsunami ate nothing but coconuts for 25 days before being rescued. So your chances are good!
Unless you can sleep, you will lose the ability to concentrate, plan and stay motivated after just one or two nights. A bundle of small branches spread on the ground and covered with palm leaves is still better than sleeping directly on the sand. After that, build a low bed frame by tying together saplings.