i have one of those things that rayray mentioned - you can get them from target - they look good and i guess are a good idea - i think we used it once or twice in the baby bath - but found we had to hold bub anyway so it really wasnt any good! and i guess it would be the same for the big bath - so i wouldnt worry about getting those lay down ones!
as for the sit up ones - we are going to look into getting one - so i will be keeping an eye on this thread