Shawn -
I suspect that there are two issues. One is the chemical content/purity of the water. Between your UV scheme and the RO filter, you probably have that one totally under control.
The second is quanty of supply. Based on my very limited experience in Hawaii, I would expect that you probably have an almost unlimited source. But the fact that you are depending on rain rather than something that is more predictable (like a public supply), there is always the nagging concern about using a lot of water for washing prints.
I have a friend whose home is on a well with an erratic track record. His approach was to standardize on RC paper in order to reduce his washing requirements. Obviously, that could be an emotional subject.
The alternative is to consider one of the water-saving washing strategies. Certainly you will want to use a hypoclear bath. But you may also want to consider batch soaking in which prints dwell in trays of water for several minutes with intermittent agitation, and are then transferred to another tray of water for another soak, and so on. A number of people have described that process here and have reported that half a dozen cycles is usually enough to remove enough residual fixer. I do this with 11x14 trays using two quarter of water per tray, so the total water volume is only three gallons.
