hiya!
we accidently ended up dealing with your situation very well! i was on a month long visit back to Canada with my ds when his sitting became good enough that we could leave him on the floor with toys and pillows behind him... like you, this didn't seem like a good bathtime position tho...
we started filling the tub less and putting Ethan on his back in the bottom of the tub and it was fantastic! he never had any water inhaling catastrophes (probably because water near his eye stopped him from turning his head too far) and he loved kicking, pushing himself up and down the bath and splashing and having ater poured over him.
by the time we got home from our visit, Ethan sat well enough that he sat in the bath with his old bath support (one of the wire ones with the cloth over it that babies lie back on) behind him and we lay him back onto it to wash his backside but let him sit with close supervision the rest of the time!
we always meant to get him a bath support, we just never got around to it and then suddenly we were coping fine without!
btw, i have heard that bumbos shouldn't be used in the bath but i did used to sit my ds naked in his when i needed to take him into the shower if he wouldn't settle to play by himself on the floor of the bathroom! he used to get a kick out of watching me wash my hair (hair goes dark, hair grows foam, hair grows dark, hair grows towel, fantastic!)
HTH!