The Munchkin Inflatable Safety Duck Tub was the first inflatable tub we purchased and it proved very handy. The inflated size is 31" x 19" so it fit well even in tight shower stalls and it was lightweight at 1.6 lbs. It deflated and folded easily and B enjoyed the duck shape. (If you press the beak, it quacks!) It even features a built-in patch that turns white when the water was too hot. The manufacturer recommends this tub for babies between 6 and 24 months, but it became a bit too narrow for B when he was about 12 months old. Of the tubs I have reviewed here, this one is the most suitable for babies who cannot sit up unassisted - when he was very small, we put B in sideways so he had little room to slide down.
The next tub we used was actually an inflatable pool - Intex's My First Baby Pool. I loved this one because it was even lighter, at only 1 lb and folded up even more compactly the the other options noted here. With a 24" diameter, it still fit in tight shower stalls (even cruise ones) and gave B lots of room to splash around. We had to stop using it after several trips when it got a hole (my fault not the manufacturer's), and I found it very hard to replace when I needed it - in winter.
That's when I bought our third tub, and now my favorite - The Safety 1st Kirby Inflatable Tub. It is recommended for babies over 20 lbs. The inflated size is 28" x 21", it folds up easily and weighs only 1.4 lbs. My only fault with this tub is I found it difficult to inflate and deflate (the valves don't let much air in/out), so it is definitely best suited to holidays where you can leave it inflated for a few days, rather than having to inflate/deflate it on a daily basis.
Please note - I have included links to Amazon for all these tubs, as I find it a helpful starting point for price comparisons and often contains the most comprehensive product descriptions. I do not receive any commissions or kick-backs from purchases of products from their website.