The simple answer to this question is: as soon as you want to. You may have heard or been told you have to wait until after your child has been vaccinated, some leisure centres may still use this guidance but the advice dates back to when the vaccinations were first introduced (as polio was very common).
The Department of Health has since stated you can take your baby swimming from a very young age and there is no need to wait until he/she has been vaccinated. There have been no cases of natural polio infection in the UK since 1984 and the vaccinations that contained a live virus (which in very extreme and rare cases were thought to have caused polio) were stopped in 2004.
Most baby swimming lessons start from around 6 weeks old, the idea of these lessons is generally to help you to be confident in the water with your baby. If your baby feels safe, he will be happy in the water from any age, as long as the introduction to water and swimming is an enjoyable experience. There is no rush to take your child swimming for the first time. Experts at the ASA and many paediatricians recommend babies start swimming at around six months old. There are other ways of introducing your child to water and increasing water confidence before his first swimming experience; during bath time ensure you keep it relatively short so he doesn’t get too cold and smile and keep eye contact, keep it playful and fun and encourage him to splash.
When you take him swimming on your own for the first time, try and do it at a time when the pool is less likely to be busy and your little one isn’t going to be hungry or too tired. Remember to stay calm, be confident, keep good eye contact and smile lots and lots. You will both have a great time. As your child gets older you may then want to consider lessons to improve technique.