I wish i'd known you could do it that young - my DD is 4 and a half months and I was waiting until 6 months to take her as that's the age i'd heard. I've been really excited about it so i'll be taking her asap
Also, this is a dumb question, but what do you do about nappies? DS was given a pair of baby trunks as a gift and I know you have to put a nappy underneath them.... Are they special waterproof nappies? Where do you get them?
You can get disposable swim nappies near the normal nappies. Lots of swimwear for babies have integral washable swim nappies inside them too though unless they're very snug I'm not sure they'd hold much.
3 months for DS1, 6 months for DS2 (because I couldnt be bothered earlier). Just go whenever you feel like it. You need a swimming nappy (pampers, boots whatever). Some pools insist on those happy nappy type costumes too as that is double protection. Have fun!
you can take them to normal swimming classes normally from about 3 months onwards, or you could do Swim Babies or whatever your local version is.
Or, you can just take him to a pool and do it all yourself whenever you want to.
One my local pools does a Parent and Baby session where the water is a bit warmer than normal and it is not full of older splashing around kids - price includes a coffee afterwards so might be worth seeing if there is something like that for you.
I did the expensive swim class thing with DD, mostly for me if I am honest, to help me get some confidence around how to handle a baby (as she wiggled more) in the water/learn some safety. But, tbh i did know it anyway, just did not realise it! With DS I did not bother, just take him for a splash when I want to.
You don't have to wait for their immunisations to take them swimming, as the diseases you're vaccinating against aren't likely to be picked up in a swimming pool.
In theory you can take babies swimming from birth, but I always found public pools pretty cool (even baby pools are only about 30-32 degrees) and so little babies get cold really quickly. It depends whether you want all that faff and expense for 10 minutes in a pool!