Who mentioned using the bathtub?Please don't use the bath tub. The litter will clog the drain. Even flushable litter is not meant for bath tub or sink drains.
I realize the deep litter might work for others, but I have found my cats prefer less litter. I keep mine no deeper than 2 inches. It does require more frequent box washes but one of my cats hates deep litter and will pee against the box or on the floor rather than step in the deep litter.
I also find that using a clumping litter with frequent scooping (twice a day, once a day at least) means I need not wash the boxes that frequently. Honestly, it's probably once a quarter, though at different times because I only ever do one at a time.
If you lack a hose, you can use a bucket!
I'm guessing you're referring to the way I used to clean my litter boxes. I don't recall anyone else here mentioning a bathtub so...
First off, I would empty the boxes totally into the trash receptacle and scrape an remnants out as well. Then clean then in the tub with hot water and dish soap. It's a spare bathroom that no one uses, but that's beside the point. It was always thoroughly cleaned afterwards.
Regardless I don't employ that method anymore. I explained what my process is many posts ago.