I suspect there will be water pools, provided the island isn't too small, and the vegetation too sparse, but it would be a good idea to take a few large water bags, to collect the rain. Ortlieb do a good line in these. There are always ways of finding water in arid areas. I even learned a new one the other day. One the kalahari bushmen use. There is another way of getting water, if you're near the shore, and the sand baking hot during the day. Build a solar still in the sand, and saturate the hole with sea water. This is really the only worthwhile application for the solar still. Elsewhere it's pretty useless, but the desert survival bags are your safest bet. You'll need to know your flora, since living exclusively off sea food is bad news. Get to know all the things that are poisonous. If it's poisonous it's probably deadly.