WDWMagic reports that as from September 29, 2013, Disney will be imposing changes to their resort swimming pool policy. Currently most pools have a lifeguard on duty between approx. 7am to 11pm. When lifeguards are not on duty, guests have still been able to swim.
From the 29th, guests will not be allowed to swim in the resort main pools when a lifeguard is not on duty. This does not however affect the smaller 'quiet pools' that are at many resorts, as these generally don't have lifeguards anyway.
These changes are expected to be in full effect across all resorts by November 2013.