Ahem... I think the main issue with the bathing question is that while families all use the same bath water, it is NOT because they are all jumping in the bath together at the same time.

Yes fathers and daughter use the same bathwater. The bath is filled at around 6pm and it stays in the bath until all the family have had their baths - usually IN TURNS.

The exception is when there are small children involved and the mother goes into supervise bath times.

Most fathers don't get home from work before their children have gone to bed. Most fathers would get home after 11pm on a weeknight.

Not many families have sento-sized baths in their houses and it is very difficult to fit more than one adult in a unit bath.

Onsens are a completely different matter and have been discussed already.