Hmm, I'm going to step into this minefield, armed only with my two years of nursing training and biochemistry.
Water is not so much an antiseptic as it is very efficient at washing majority of germs away from intact skin. The cleaner the water available, the better this works, obviously. They do use sterile water in hospital to clean wounds too. Often this is all they use depending on the wound.
My point was only this: not that water was antiseptic, but whatever cleansing agents they possessed back then, including dirty water, spit and/or camel dung, the risk of infection would have been astronomically higher once God had instructed them to cut the skin. So the argument that all knowing God asked them to do this for any type of hygiene reasons makes no logical sense.
The doing this to make them a separate distinguishable people makes more sense. So God asked them to put themselves at risk to do this.....or they thought it up themselves to give them a tribal identity, knew nothing about the risk of germ infections from open cuts, and claimed divine insight from God. I favor the latter option myself.