The 2.3-kilometre circuit which some swam twice today took them around the iconic site noted for the needle-thin golden church spire that rises 122 metres high.
The cathedral is where most Russian czars since Peter the Great are buried.
It is a tough swim not only for its length, but because of strong currents and seaweed in the Neva River.
Yelena Vodolazova, 46, of St Petersburg says "it was not only a physical challenge for oneself but also an unforgettable experience of swimming in such beautiful historical location where swimming is normally restricted.
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