In sPvP, Cleansing Water only procs once every five seconds (wooo for nerfs ’to encourage diversity’, which pushed even more people to cantrips for survivability and didn’t stop people from taking 20 water).
This means that you’ll get double cleanse at the start of Healing Rain, the second regeneration won’t proc CW (though you’ll get the regular cleanse from the skill), and the third one (just as the field disappears) will proc and give you a double cleanse.
If you switch to water attunement and have Elemental Attunement going on, though... you’ll likely not get cleansed for #1 and #3, since you procced Cleansing Water on switch.
It’s rather a waste in sPvP now - and it removes one of the few useful effects of Inscription, since you can’t chain glyphs for cleanses or even get double cleanse from Glyph of Elemental Harmony (which also doesn’t work if you switch to water to use it while traited with Elemental Attunement).
EDIT: fixed a couple of typoes
