Potable water defined as (suitable for drinking because clean and uncontaminated) is abundant and free for the taking anywhere it rains. All you have to do is catch it as it falls from the sky in its purest form, particularly in the South Pacific islands where there are lots of tropical showers and little to no industry to pollute the air.
The simplest system is hardly a system at all, just a tin roof and a drain pipe to a large fresh water holding tank. From the tank you run a waterline to your building. The entire country of Tonga uses this method to supply their "potable water."
You can locate the water tank on higher ground so that gravity feeds down the pipe to your home or resort. In this way you need no pump to deliver the water and it can come in high pressure for showers. More convenient might be to locate the tank(s) right at the roof source of your building. Even this provides a gravity flow when the tank is full. An electric pump, AC or DC can be installed to boost the pressure to your liking. The popular pumps automatically turn on and off on demand.
For those with germ phobias, you can add a filter system where the water comes into the house and be sure of even more purity.
The supply and demand matter can be worked out as you go along by adding more tanks and adding more capture from more of the roof area. You can even get scientific about it and calculate how much roof, how much rainfall and how much storage you might need by calculating your usage. Most folks just look around to see what the others have that works fine and just copy their neighbor’s set up. If you over do it, not a big loss since you have overflows built in the tanks and that flow can even run down to a garden below. If you calculated not enough storage, just add more.
In the South Pacific there are two kinds of water storage tanks, the store bought food grade that come in all sizes, plastic and cheap. The other popular tank is a "ferrocement" tank made on the spot with wire mesh and cement applied like how they make ferrocement boats, just two inches thick and strong as any tank out there.
