We based ours on recommendations from other mums. Also looked for one that was heated/hydrotherapy so that we could continue classes through the colder months. After a couple of terms you came to recognise which teachers were better then others and hence why their class times were always so hard to get into! But it is all a bit trial and error.

DD started from 4 months old and was a real natural waterbaby, no fear etc. She also had a bath seat - which I know that some people say inhibits them when learning to swim but we didnt have a problem at all.

DS on the otherhand hasnt started formal lessons as where we live they dont offer them. But we take him swimming as often as we can.