Why does St. Isaac's Cathedral stand in water in this painting?

The young artist had recently moved to St. Petersburg, hoping to enroll in the Academy of Arts. His idol was Ivan Aivazovsky: As a 13-year-old boy, Arkhip had traveled from Mariupol to Feodosia to ask to become his apprentice. Thirteen years later, now in the capital, he was enrolled as a student in the great marine painter's school and began painting landscapes in a similar style. Among them was the painting ‘St. Isaac's Cathedral by Moonlight’.

The moon peeks through the clouds in the night sky, its light shimmering on the surface of the water as a boat glides across it. The cathedral itself, however, is immersed in darkness. But, why is there a whole lake around it? Kuindzhi was trying to capture the aftermath of flooding that frequently occurred in St. Petersburg. The Neva River overflowed so much that it flooded the square around the cathedral, making it impossible to navigate except by swimming.