Our big bath is too deep for me not to get in with DD, once she outgrew the baby bath we borrowed our cousins plastic baby support, long seat thing and filled the bath big enough to cover the bottom of it and we hoped in with her.
Once she was capable of sitting on her own we have the suction cap round bath seat, she LOVES it... is just starting to explore the bath without it now but very slippery!
She refuses to lay down in the bath now though, just wants to be up and about!