C's tub is large enough for it. We tried once, but it didn't quite work out, so the tub is for After Sex Zen Time. I tend to zone out after yoga and sex in the evenings, so I'm pretty mellow by the time we get into the bath - and he's always mellow.
Tub here is wayyyy too small. It would never work. Honestly, while the idea seems romantic, it really doesn't appeal to me at ALL realistically. I'd rather start foreplay in the bath, and then progress elsewhere.
My Master is a bit on the larger side, and there's not enough room for us both to have sex. However, we have snuggled in the tub on numerous occasions, and I've given him blowjobs in the tub many, many times!