Nope not been to lock 21 yet. I've only dove Ivy Lea, Centeen park, and Cardinal (the Conestoga).
@mddolson was recommending the Wee Hawk. Takes a little organization but I like doing Wee Hawk, Lock 27, Conestoga.
You need to go to the Legion Way. Drive pass the Legion and down the peninsula. Just before the peninsula narrows down, you can park a car on the side, off the road.
Leave one or more cars on the peninsula (near the Conestoga). Drive everyone back to the Highway 2, go left toward Galop Canal Road. Turn left onto Galop Canal Road until you get to the bottom. There will be parking on the right (and an outhouse). Walk across the foot bridge, over Lock 27. Walk to the right and you'll find the remains of Lock 28. This is a good spot to walk into the water. The Wee Hawk will be right there. It is sticking up out of the water. So you can see it.
I would then go to the Wee Hawk, explore it, go north to Lock 27. Drift down Lock 27. Keep to the bottom and close to the south wall on your right. This way you avoid boat traffic. I don't bring a float and flag. It is just something for the boats to hit.
As you drift down Lock 27 (around 15 feet), there will be an opening on the right (south) to the St. Lawrence. The current will want to pull you through the opening. Let it. It will get a little shallower than 15 feet. Once you clear the opening it will get deep again. Go to 20 feet and let the current take you to the left (east). Stay at 20 feet and it will take you to a radiator with a plaque. There will be a line on the radiator. Follow the line to the Conestoga.
You want a car at the Conestoga so when you finish the drift, you have a car to take you back to the other car at Lock 27.