I finally invsted in some glass/stainless steel water bottles for my rats. I have been buying about $20 in replacement plastic bottles a month it seems, from them chewing them. I filled them everyday and provided chewtoys, etc.. to no avail. These ones seem very nice; made by the Petland brand name, and they were only a few bucks more than the cheapo plastic junk.
I use tanks, so I could not place the bottles outside the tank.. except for one, where I use metal grilles as a lid.. they can't chew that sucker!