You have plenty of time to see the Falls, go on the boat, and cross the bridge to the American side if you like.
I would recommend renting a car from the airport, if you can drive. I'd expect to pay 40 CAD for a one day rental plus fuel.
It's probably a 90 minute drive in good traffic. At 6 am it will be longer as you meet rush hour traffic, however some of it you are going against the flow. The falls will be quiet when you arrive. Your return journey should be smooth if you plan to be back at 4.30.
If you want to do it all on public transport you'll probably need an extra few hours beyond what you have there. I'll leave that answer for someone else to give.