re: Watery Mars vs. Stuffy (CO2) Mars vs. The Moon...They are all nuts!
"And didn't somebody postulate that there would be huge amounts of water on Mercury as well?"
"...radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters."
http://seds.lpl.arizona.edu/billa/tnp/mercury.html
I'm not sure if anyone has actually said how much quantity of water is suspected as being on Mercury's poles. Since Mercury is much larger than the Moon and because it as other factors which may increase the amount of water, I would guess it's somewhere around the tens of billion of tons range.