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Thank you for your question. My name is***** I have been a small animal veterinarian for 14 years and I'd like to help with your question.
So, many cats drink a surprisingly small amount of water. If you are feeding a significant amount of canned food then this might be okay. They do get water from their canned food and if they are eating at least half their diet this way it can be enough. However, if they are drinking a completely dry diet and not drinking they will become dehydrated. Typically if they are dehydrated then providing them with fresh water is all you need to do, they will be thirsty and drink, but if that doesn't seem to work there are a few other things... you can try moving water. Some cats will only drink moving water. You can find cat water fountains in most pet store... it constantly circulates the water so it is moving and encourages cats to drink. You can flavor water by adding some chicken broth or the water from a can of tuna. You an also just give them more canned food if they like it.
As for the panting outside.... that is a problem. Can't don't pant unless they are very stressed or have some kind of lung problem (asthma, heart failure). Cats don't pant just because they are hot. If she is really breathing with an open mouth an any time then I would have her checked. She will probably need an xray of her lungs.
I hope that helps, but if you have more questions please let m know.