i never let my son went for those baby swimming tub lesson, in fact i find it is quite ex, wanted to buy a swimming tub n neck float and diy at home but did not also. In the end, I brought dear son to swimming pool when we were overseas in hotel's pool, he is about 4mths+. I bought a ZOGG trainer seat which he sat on it soo steady & comfortably, i am happy to have bought this afterall.
As for swim diaper, i never use it. i just strip him off and put on his O'neil swimming suit.