we call it pool tax for simplicity, but pools are not taxed per se.
the fact is that if your pool is undisclosed, you haven't requested the necessary authorizations (what you actually pay for), which means it is not deemed safe.
Pools are taxed directly when they are considered a luxury item in a house that have all the features (there's a list) of a luxury residence.
It's already a progressive tax, it couldn't be any different here, progressivity of taxation is written in our Constitution.
There's also that pools are not a free imposition on the community. There's a regulatory apparatus that must be followed - i.e. appropriate child-safety fencing to ensure children do not drown, and your neighbors have a right to not have it built in a way which impacts their property (i.e. proximity to fencing boundaries).
If water consumption is a problem tax it progressivly.