The most likely cause of a watery eye in cats is conjunctivitis. The eye reddens because of the increased flow of blood and swelling of the mucous membranes is sometimes seen. The most common cause of conjunctivitis is herpes, a virus that is sometimes compounded by a secondary bacterial infection. There are several treatments for herpes, antivirals such as idoxuridine, trifluorothymidine, and vidarabine can be administered as eye drops. Rinsing your cats eye with freshly made room temperature saline (1/4 teaspoon of salt to 1 cup of water) will also help if your cat will allow it.