Assuming it's a tropical island, get some banana leaves and layer them for support. Set them inbetween some rocks to prop them up a bit. Rainwater collection.

You could also make a kiln with mud and sticks and leaves so you can fire a clay bowl for boiling water from freshwater sources like springs.

give us more information. Is this in minecraft or reallife? What do you mean with 5 natural material? Are you allowed to use buckets?

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