Sudden onset or temporary clear discharge from the eyes can be caused by injury (although this is unlikely if the discharge is from both eyes), allergic reaction or an upper respiratory tract infection - just like when humans get watery eyes when suffering from a cold. If your cat has a respiratory infection he is likely to also develop a wet nose. If your cat has become lethargic, or there has been any change in his toilet or eating habits it is likely that the wateriness is just an exterior sign of an underlying illness.
I'm just curious..Are the cats current vaccinations up to date? Because generally if they are your cat (unless it is allergies) shouldn't have any respiratory infection. Nevertheless if your kitty does have a kitty cold that includes heavy sneezing, drippy nose and eyes this will most likely clear up on it's own, generally within 1-2 weeks. As long as kitty is eating, dinking and pottying (urinating) then there isn't really any concerns. If kitty is lethargic and not eating/drinking call your vet ASAP to get more information on what to do. Once again look to see if vaccines are due because often that's the issue. The vaccinnes cover againt several respiratory germs! Hope kitty feels better!