It's also very dangerous to swim in the canals because of motorboats and gondolas circulating the canals constantly and at a rapid pace. Not even on the so-called fondamenta (the seashores) are you allowed to swim. It is even more dangerous, since bigger boats regularly stop by.

No, it is strictly prohibited and highly unsafe to swim in the canals of Venice. There are three primary reasons for this ban. First, the water is unsanitary; while Venice has modernized with septic tanks, the ancient city lacks a modern sewage pipe system, and some wastewater still flows into the canals where it is flushed out by the tides twice daily. Second, the traffic is dangerous; the canals are active thoroughfares for gondolas, water taxis, and heavy "Vaporetto" buses that have significant blind spots and cannot stop quickly. Third, the fines are severe; if caught, you will face a fine of at least €350 (approx. $380) and a "DASPO" (an immediate expulsion order from the city). If you want to swim, the locals head to the Lido di Venezia, a nearby barrier island with beautiful, sandy beaches and clean Adriatic seawater, which is just a 10-to-20-minute water bus ride from the historic center.