Thanks for the questionCustomer
While I cannot diagnose this over the internet I am more than pleased to talk over what would be going through my mind if I was presented with such a case. Although the coughing is of concern in a dog of twelve and may indicate serious disease like heart disease you have to methodically start to diagnose the problem by focusing on the main symptom which in your case would be the excessive water consumption.
I will list the conditions below which would commonly make a dog drink excessively in my experience, there would be others of course.
1. Diabetes mellitus - sugar diabetes - easy diagnosis via blood test
2. Diabetes insipidus - rare but does happen - catches vets out !
3. Kidney problems - diagnosed by blood tests
4. Chronic hepatitis - diagnosed via blood tests
5. Cushings syndrome
Here is a good link on the subject so you can look at things in depth : LINK
As there is no scope whatsoever here for home remedies all I can suggest is that you have your vet check your dog out and perhaps carry out some blood tests, it would be helpful if you take a fresh urine sample with you as there are stick tests which might help your vet pin down any diagnosis more quickly.
If I have not covered your question fully enough or you would like to ask more I will be online for the next hour or so and I will be at your disposal.Scott Nimmo BVMS MRCVS