this seems like the most common subject! If you do a search for it, i'm sure you'll find tons and tons of older posts about this.
we got married july 15th 2010 at CN at 10am, i'm happy i chose that b/c the water was the most crystal clear blue, weather was slightly cooler, and we missed the rain! That day it rained from 1-3p, and again from 530-9p
Our wedding is 03/21/11, and we chose 11am. I know the water seems much prettier in the morning. Plus, I didn't want it to be too hot, and we wanted to have a nice relaxing breakfast before I had to get ready. Also, less chance of showers in the morning, normally. I'm sure whaetever time you choose it will be beautiful. :O)