St. Petersburg was built on the Neva River and its tributary, the Fontanka. The Neva has been integral to St. Petersburg's history, architecture, and transportation. Almost 350 bridges have been built in St. Petersburg alone, not counting those bridges in its suburbs.
If you have the opportunity to take a boat tour through St. Petersburg, certainly do so. Along the banks of the Neva, you'll see the facades of buildings that overlook the river. These buildings were once the homes and offices of St. Petersburg's notorious nobility.