Staple costs

Off out to FP in a couple of weeks for an independent jaunt around a few islands primarily with diving and lazing about on my mind.

Am currently working out finances and how much cash to carry when I visit smaller islands, so am trying to get an idea of the cost of the staple items such as bottled water, bottles of beer, food etc. so I can get some perspective.

If anyone can enlighten me on this it would be appreciated.