Rain water? What's rain water (in northern Florida)? The well water in my new north Florida location is 250+ TDS, pH 7.8 - 8.3 with high [iron]. Not wanting to use Na softened water, I used the well water. After a year of poor plant growth, some of which might be attributable to water quality (hi pH and TDS), I switched to KCL in the softener and began using softened/treated water. I also purchased a pH meter and use "Downer" to adjust the pH into a range (5.8 - 6.5) suitable for the plants to make better use of available nutrients.
I have a small 25x10' enclosed green house with about 150 generally hearty plants in only a few genera (nothing exotic). Even having switched half the collection to S/H, I still use 25 gallons of treated/adjusted water at a clip. In between waterings and cooling is done with straight treated water.
If the collection were smaller or I was indoors, more expensive water alternatives might be an option. I tend to grow what grows for me. As my facility conditions and techniques get better, maybe the collection will diversify accordingly.