I see a few questions and I hope that I can answer them together. Let me know!
If a mammal drinks too little, then as long as food consumption is normal, the animal is consuming enough water for hydration purposes. While it is thought that some pets, especially male cats will benefit from more water consumption than less; as far as the purposes of existing disease
less water consumption in the face of a normal appetite is not relevant.
Can an animal be normal and
drink a lot of water? In short, yes. If PU/PD exists, then the water consumption is always
elevated and the urine is always
dilute. If the urine can be concentrated, then these diseases do not exist. This is why a first in the morning urinalysis can be so useful.
Hope that helps.