Cool off without fighting the crowds in the centre. These sit out on the edges of the city, so pick the one nearest your route and make a stop of it. It is a pick-list, not a route: one good swim is usually enough for a day.
Houthaven gives you a sheltered dip in the west, and Nieuwe Meer is the big open water in the southwest with room to anchor and swim off the boat. Down south there is the Amstel swim spot at Bajeskwartier for a quick one off the quay. And up in Noord, Café de Ceuvel doubles as a swim stop: tie up at the dock, swim off the deck, then dry off with a drink right on the water.
A few honest notes. Open water is swim-at-your-own-risk: mind the boat traffic, keep clear of the fairways, and check the water quality before you go in. The city posts it for the official zwemlocaties, the marked swimming spots. Cold water and currents catch people out, so know your spot and do not swim alone.