Normal for me & DH to share bath water. (We just ask if the other wants the water b4 we empty it.)
We generally bath cos we enjoy the hot soak, with a book or music. (Rather than to get some dirt off, IYSWIM?)
Humans can't live without skin/gut bacteria, & vast majority of 'germs' are our friends, so why are ppl so obsessed with "cleanliness' all the time? And when u live with someone, (partner/kids), u prob all share the same 'germs' anyhow? (Which is why "fecal transplants" are usually taken from a partner if poss/required!)
I get cross that most ppl expect to have/use lots of hot water any time they like with no restrictions? (Then they complain about size of their bills ... pfft!) This is the "I want it all and I want it now, cos I am worth it" modern mentality?
My parents said they shared just a few inches of hot water in the bath per week, (their weekly allowance), when they were just married, (in the early 50s), cos they rented a bedsit, (bedroom), in a widow's home, as many ppl did back then, where the, (always strict), landlady ruled the roost/made the rules. She stupilated how much much water they could have for their "once a week bath", cos once a week was the acceptable freq of bathing back then. (And there was no shower, just a cold strip wash the rest of the week!) Lodgers couldn't complain and were just glad to have a roof over their heads at a reasonable price! (While dreaming of renting their own little flat, once they had saved up.)
It was completely 'normal' in UK, (after the war), to be so frugal, cos most folks were brought up to be v careful with money, and 'excess' use of anything was considered a personal weakness/sin, rude or just foolish. (How things have changed?!)
My verdict: keep sharing the bath water! (It's both economical and environmental. And probably helps your immune system keep strong?