Hi there Ben. Certainly use ...

Hi there Ben. Certainly use of polyethylene is now well established and it can be gradually sunk by attaching concrete weights occasionally. You are going to need a flow of around 100 l/s as a design and will need storage on the island to even out demand of some 10,000 m3. You might look at tankering as an alternative. This is used quite commonly and tanker costs are surprisingly low. I know that water is taken by tanker from the US to the Bahamas for example. Also consider how you might limit demand or exploit reuse / treatment on the island. Hope this helps?