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.