There is this product you can get (we haven't got it yet, but i think i'll get it this week)

Its by Infantino, is this thing you put at the side of the tub, it has a place for you to sit, and you kneel on pads, and there are elbow rests too. Its on sale at Toys R Us this week (if you look up their catalogue online you can see a picture).... Can't tell you if its good or not, but DH back gets really sore giving baths (our bath is broken at the moment, so we are doing the shower thing, but IT WILL BE FIXED BEFORE I GIVE BIRTH (hear that Rob???)) , and it looks like it might help?

My son loves having pretty deep baths and splashing around a lot, even when he was like 6 months old, which makes it hard cos you have to hold them and not take your eyes off for a second..

It the beginning we also did the 5cm thing, which worked well.

Good luck