Just because it's water doesn't mean it doesn't have a COSHH data sheet.
It could be some sort of weak acid solution, not water I'm using, for all they know?
But if you google, I can find information similar to a COSHH data sheet, but it's not exactly what I'm looking for; though it will fit the bill.
I can't be the first window cleaner to be asked for COSHH details; in fact on my method and risk assessment I say that I can provide them on request (
).