I took my baby girl swimming for the first time when she was just 5 months old, (3 months corrected - but I don't correct her age anymore as she just acts like any 5-6 month old). She weighed in at only 11lbs but it doesn't matter. We bought that seat from Mothercare, but she was only in it for about 5 mins then wanted to be out in the water with us. She quite enjoyed herself. Just take your baby whenever you feel they're up for it x