Kia Orana Sara,
Welcome to the forum.
In all my time spent on Rarotonga I have never ventured to drink the tap-water straight from the tap. Inspite of having a water filter on my tank I continue to boil tap-water for 3+ minutes or longer. I also go to the water treatment outlets on Rarotonga to stock up on filtered and treated water.
Frangi in Nikao, and Vaima in Avatiu are treatment plants, where you can purchase filled containers and get refills. All the small stores and the bigger supermarkets on Rarotonga sell water. You can also refill containers at Wigmore's in Vaimaanga.
A tip when dining out, it is likely that when you request water that you will be charged for this service. Ice in drinks is more than likely to be commercially-made ice cubes.
Have a lovely stay. Let's hope our long spell of fine weather will hold out for your visit. Hardly any rain over the past 6 months!