If water is your only concern, and assuming there are no fresh water pools on the island, then you're in luck. Take 3, or 4 if you can afford the weight of what Aussie bushmen call desert survival bags. These bags are in clear polythene, 600 gauge, dimensions 2m x 1m. You can read up on how best to use them, if you're not familiar. A word of caution, don't go using them over poisonous foliage, and take a roll of clear sticky tape in case of punctures.