Impossible to Swim Properly in the Summer

It’s really impossible to swim properly in a public pool during summer. I usually swim twice a day, but with so many people and a large number of slow swimmers, it’s impossible to get a good workout. I try to swim sets of 50, 100, or 200 meters, but I’m often stopped before even reaching 25 meters. What can I do to still have a good swim despite these difficult conditions ?