What should my mum put in her bath water if she has a water infection?

A sitz bath with epsom salts or baking soda and perhaps a drop or two (no more!) of tea tree oil might be soothing. And consider having her take cranberry capsules with d-mannose.

What is a water infection? Are you speaking of a Urinary Tract Infection? NOTHING should be put in the water of an elderly female because most things contain fragrances. The mucous membrane that protects the thinning walls of the urethra of the female are very susceptible to infection. Any products can constitute an irritant that can then lead to inflammation and infection.

Hornets?
Do not put anything in the bath water of a patient who has a bladder infection. Discontinue bubble bath soaps.
She should be showering instead, keeping clean, rinsing well.
She should drink lots of water.
Taking her prescribed medications.

Is that what you meant to say when you called it a water infection?

Never heard of a water infection.

What is a water infection? This is something you need to ask her dr.