On-site at the Aulani, they have those Coke machines in several places that you can use to refill water. There is a specific water button on them, and from my understanding those have filtered water. We drank glasses of water at most of our meals (both on-site and off) and it tasted just like it always has.
Pre-pandemic Aulani used to have water stations at the front of the hotel and down by the lazy river that were ice water in a dispenser, with some fruit in it. Unfortunately, those have not made a return (yet, maybe never) but they did put a bottle refill spot (like a water fountain, but for bottles) at the old lazy river spot.