Lynn- Ty will go from sitting to all fours, but it's more because he can only sit for a few minutes of a time and he then puts his hands in front of him and walks himself down to all fours... I think just a swim diaper is fine! But target also has bathing suits still, at least near us they do. Did you use a pool noodle to install the carseat to get the right recline?
Tips for Installing Child Car Seats