I've made numerous trips from Rochester to ports across the lake, and would recommend Cobourg (even though it's a little west). Its a nice little town with a couple good bars/restaurants and a nice beach.
The other nice option is to go directly across and enter the Bay of Quinte. Once you go into the Bay of Quinte, after passing through a canal a couple miles long directly off the lake, you have access to a few ports (Trenton, Belleville and Picton are a few of the larger ones). Its a very nice ride through the Bay of Quinte, and you could even drive all the way across it and come out the east side then head back across the lake.
The biggest thing to be very cautious of (other than the canadian regulations others pointed out) is how fast conditions change out ther and how often weather, especially wind forecasts are wrong! Its about a 60 mile trip across and a lot can change in 3 hours. I'd recommend planning an early morning start after a calm day/night before and be able to stay the night if the weather changes after you reach the other side.
Good luck and enjoy. Trips across the lake have been some of my best boating memories!