My Sims live off the grid. Unfortunately, there’s no plumbing which makes all tubs non-functional.
It’s does work quite well to take a shower – except for toddlers and pets. They do still get dirty and I have no idea how to get them clean again, since the option on the tub is grayed out.
Although the option on tub is gray, it does still work on the toddler or the adult.
Click with the adult on the toddler and choose Give Bath or click with the toddler on the adult and choose Ask for bath.
Pets work the same way.