Sailed on Lake Champlain for years. Wakes from the ferries are no big deal. On the other hand a good blow from the South will show you waves which are ten feet peak to trough. I''d rather not take those broadside in a 21. The good news is there are so many sheltered anchorages around the lake you can always run for cover if need be. I loved sailing there on Redline my Jeanneau 45.2 and I'll get back there in my R 31 I suspect.
One last thing - make sure you have a weather radio, Lake Champlain is one of the thunderstorm hot spots of the US, second to Miami! I admit that I had a few scary moments avoiding tornado warnings which were quite frequent.
Got hit by lightning once - lost everything electronic on board including an ammeter which wasn't even connected.
With the huge number of sailboats around, I suspect a power boat is likely to be protected by the nearby masts.
Enjoy, I think a 21 will be fun on Lake Champlain and a nice difference compared to the number of floating condos operated frequently by people with few skills. I was once hailed by a 40 foot powerboat off Plattsburgh by a guy who asked how to get to Mississqua bay. That isn't a simple route and the guy had no charts!!!
I like to think that tug boat people are more like sailors 🙂
Best,
Bob