I just let my 10 month old sit in the tub. You just want to make sure you have a grippy pad on the bottom of your tub or a none slip tub. I fill the water up to my babies belly button.. maybe a little higher
@Sophia what kind of bath mat do you have?
I have no idea about bath seats but Once mine was able to sit pretty well on her own I just started letting her sit in the tub with enough water to cover her bum. I used to cup to help her hair wet and then would lay her down on her back to wash her hair or she comes in the shower with me. Hope that helps