I am travelling with my family to Prague in August. We all love swimming outside either in public pools or lakes especially if is hot. We find it is a good way to brush shoulders with the locals. Where would people suggest we go.
If you are restricted to public transport or taxis then Podoli has both inside and outside 50m pools https://pspodoli.cz/en/. If you are driving or prepared to go further out then do a search for "venkovni bazeny praha" and look at the options.
Yes we will be restricted to public transit.