People are countable. That's the whole point of a census. Water can come in any amount, such as 3.5 litres. People come in discrete numbers. You can't have .5 of a person.
I think the thing confusing you is that it's one of those weird English words where the singular and plural word are different. Person/people vs car/cars.
You can still say "waters" though. For example: "The waters of Amsterdam" would collectively refer to all bodies of water in Amsterdam. Another example I came across online was "Don't forget your waters" which referred to multiple bottles of water.
Uncountable nouns (i.e.,things you usually measure by volume or weight) behave this way as well.
e.g., salt/salts to mean any measure of salt vs different types of salts. Beer/beers to mean any amount of liquid beer vs different beer styles or products.