I use this to take baths together with my 7 week old daughter.  It's wonderful, and holds the poop in.

I got the small size and it fits her nicely.

(For safety, I line a bassinet with a towel and put it right next to the tub.  I put my daughter in the bassinet.  Then I get in the tub, then I reach over for her and bring her in with me.  When done, I place her back in the bassinet before getting out myself.  This is such a great thing to do with your baby.  I nurse her while the rest of her body free-floats.  She's so relaxed and content.)