The water quality in Harbour Island is not the best city water, it is brownish and has a salty odor.
The large majority of hotels and upscale vacation rentals in Harbour island have a reverse osmosis plant also known as desalination.

The process is simple and complex at the same time:
– A deep well needs to be dug
– Pumps need to be installed to bring the water to the desalination
– A system of filters with membranes remove the salt and all impurities
– The purified water flows into 2 tanks

The water becomes now drinkable with absolutely no taste. The process needs to be monitored carefully as well as the quality of water after the process.
Hotels and luxury vacation rentals that have a well monitored reverse Osmosis system should tell you that you can drink the water, however they may not do so because they don’t want to take the liability or because they want to sell you fiji water at $ 10 a bottle.
You can always ask for tap water if you stay in a luxury establishment, they will not serve you if it is not safe and will tell you.
Desalinated water is used for showers, kitchen and used in washing machines.
If your hotel or vacation rental does not have such a system you should absolutely not drink the water. You will find plenty of drinkable water at the local grocery store, the treated Bahamian water is perfect you will not need to pay for an Evian or Acquapanna water.