Should you drink the tap water in St john?
That is definitely a question to ask the property management of the place you are staying. Hopefully your villa will have a water treatment system.
However, some rentals will tell you to buy bottled water. If it is not somewhere in the description of the villa, be sure to ask ahead!
It’s important because water doesn’t come from a city treatment system or from a well. Water on St John is stored on the property site and likely comes from the following sources: either rain water collected at the villa (collected from the roof during rain then stored in the cistern); or from water that is delivered (desalinated salt water) which has been trucked to the villa location and emptied into the villa cistern.
Ideally, all homes would have water filtration systems, and the question about if the tap water is safe to drink at the villa would be solved. This may include several filters and or UV light that the water travels through before reaching the sink. Using the water to shower should be no problem. However, be sure to ask if it is ok to wash and brush your teeth, cook with the villa water, etc. because not all villas have potable water.