I think of you’ve agreed a payment plan and then dont pay it you are in a worse position than just not paying it in the first place, as by agreeing to the payment plan you agree I’m the debt is yours and you can afford to pay it. Disputing the debt until it goes to court gives you a stronger position i believe.

Main difference from not paying your gas or electric bill is that they can’t legally cut your water off.

Obviously get proper legal advice as I’m far from certain or a lawyer.