Is anywhere in the Brisbane area more than 45 minutes from a cold body of water suitable for swimming in? I don't think there are crocodiles near Brisbane.
The sea is about 45 minutes away but only rock beaches.
But we have the famous beaches of Sunshine Coast (Noosa) and Gold Coast (Tallebudgera amongst others) around 90 minutes away.
Though I can guarantee on a hot day, 2.5 million people aren't making their way to beaches or water holes to cool off. And Brisbane is a sweaty place in Summer.