As a GM, I'd rule that things like oceans, lakes, rivers, etc couldn't be turned into holy water, there's too much constant interference with the water--tides, sediment, animals, etc. One might be able to bless the contents of a water reserve, but many sprinkler systems use the water within their jurisdiction, so it'd be a little harder to bless or maintain such a blessing because the city/county/whatever's water would be immediately dispersed amongst the greater populace.
A tactic I took with clerics in my D&D days was that holy water wasn't just a blessing, it required sacred oils to help the "flow" of that blessing. It wasn't exactly chemistry, but there was a minimal amount of oil-to-water in order for the holy water to work against creatures of the night. Blessed water without the oils was used for ritualistic and spiritual purposes only.