It was quite usual for men to swim in the nude then, at any rate when there were no ladies about.
And even when there were ladies about. Olga Alexandrovna tells as nice tale of how she and her father and Mikhail would quite often come across soldiers swimming in the nude when walking in the park at Gatchina. On sighting the Tsar, there would be a mad rush to get out of the water and grab their caps, so that they could fall in and give a smart salute - wearing nothing else!
Ann