Can I swim at Niagara Falls?
No, you absolutely cannot swim at Niagara Falls. It is illegal, and it would kill you instantly. The currents are insane, the water is freezing, and there are massive underwater rocks and whirlpools. Even the calm-looking pools below the falls have a deadly undertow. Don't even think about it.
If you want to get wet, do the Maid of the Mist boat tour (about $25-$30 per adult, runs April to November). You'll get soaked in the mist from the bottom of the falls, and it's the closest you can legally get. Or try the Cave of the Winds tour on the US side (around $20), where you stand on a wooden deck right next to the Bridal Veil Falls and get blasted by spray. Wear sandals you don't mind getting wet.
For a real swim, skip the falls and drive 15 minutes north to a public beach on the Niagara River or Lake Ontario. But honestly, the water is still cold and not super clean. Just stick to the boat rides and save the swimming for a hotel pool.