If you want to swim centrally, then this is a great place, close to the centre, and outside a cool cafe with drinks and food!  To get to the KNSM Island, either take the 26 tram or 48 bus from Central Station or the 65 bus from Amstel Staion and get off at the Azartplein.  Then walk west along the Levantkade to the Kenis em Meiland Cafe at number 127, see map below.  There is a deck to dive off and soak up the sun on.  The water is clean and fresh from the Ijselmeer!