You cannot have your eyes on every child at all times, even in a nursery. You look away, tend to other children, fill in records, wipe tables, get toys out. You may be in the same room but every child does not have an adult's eyes glued to them.
In the same vein, a parent cannot function without taking their eyes off a toddler. You have a safe environment so that you can cook, get dressed, go to the loo, make a cup of tea, etc.
In recognition of this fact, there are laws in many countries where swimming pools are common requiring home owners to have pools securely fenced. Children can slip away from adults very quickly and easily, even if that person is in the same room and is aware that they are responsible for them.
Being near an unfenced pool, e.g. at the swimming baths is different. You can maintain a level of supervision for that short time that isn't possible 24/7.
Where swimming pools are in a home, it is absolutely necessary to maintain your usual levels of supervision plus ensure that the child can never have access to the pool for even a few seconds unless they have 100% of a responsible adult.
This lesson has been learned the hard way far too many thousands of times. It is perfectly reasonable to expect every adult in a house with a swimming pool to behave responsibly in order to keep a toddler alive.