Glad to hear you already have a vet appt.

Please make sure they know that you are having the water drinking/peeing issues BEFORE your appt. Vet's schedule appts based on how much time they think they'll need and it sounds like they'll likely need a bit more time with your puppy than the standard 1st puppy visit. Don't be surprised if they don't do vaccinations that visit if your puppy is ill.

I hope it turns out to be something easily fixed and your puppy will be just fine.