Thanks. I had a feeling. It's one of those instances where the お has "become" part of the word over time, like in お茶 and, to a lesser extent, お金.

Don't many women say お水 to sound a bit more feminine? Or am I misremembering? I think I remember being taught that, but I'm not sure how often I actually heard it when I lived in Japan.

Well, women do but so do men in certain situations.

In restaurants, almost everybody says お水 regardless of the gender, but no one uses the お to refer to the water in a swimming pool or a flood.

When talking to babies and small children, a lot of men use お水 as well. Both men and women tend to speak extra-politely to small kids. e.g. おともだち (friend)、ごほん (book)、おさら (plate)、おえかき (drawing)、おかたづけ (tidying up)、etc.