Not on this planet... most rain is caused when moisture condenses around small particulates in the air... that would tend to indicated to me, that the rain has a fair bit of particulate matter in it. This doesn't even address issues like chemical pollutants, like sulphur dioxide, one of the primary components of acid rain, volcanic ash, airborne dust from drought stricken areas or a million other sources of pollution. Airborne pollution can travel long distances, so the water from rainstorms over the open ocean isn't much better than that near coastal areas. Go ahead, get a bucket and catch some rainwater in, and cover it and leave it sitting in the sun for a week. It won't be the "clean" water you think it is after that time is my guess. Plenty of pollen, microbes, mold spores, etc in that rain water.
What you say about rainwater is true but how do you think it compares to water at many foreign and some domestic marinas?
"When in command, command." -- Admiral Nimitz
Difference between a power boater and a sailor out on the water: A power boater is going some place special, a sailor is already there.
s/v Zotz 1981 Pearson 365 Ketch Hull #375