Many cockers love water, it's true, but as far as I'm aware it mostly involves splashing about in it rather than drinking it to excess. I'm glad that kidney problems have been ruled out. Is she panting as well? Have they also ruled out diabetes?
It could be many things and it seems your vet is ruling things out one by one which is encouraging. She could be drinking more because of a temporary infection for example.
She has been on dry lite food now for well over a year (a bit overweight)
Spayed bitches, as you probably know do tend to put on weight more easily as a rule. If she is eating dry food then she will be drinking more than if she were eating wet of course. To help her lose a bit more, can you get her excercise levels up and cut out any highly processed treats and give her raw veggies instead perhaps, such as carrot or apple? Check her current food does not contain uneccessary additives/colours and that it is a good quality food. She could be drinking more as a reaction to some additives
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Hope you get to the bottom of it soon. Do let us know what the outcome is.
