I know this is a late reply, but i'm relatively new to the forum. You should check out Winnebago, she's usually a relatively calm lake to sail on. The water clarity can really degrade depending on the weather throughout the summer, but it doesn't stop us from sailing once a week at a minimum. I sail out of the north end of the lake, but you could conceivably sail from Fond Du Lac to save on some gas. Just watch the wind reports, once you get a decent breeze the waves can pick up in a hurry. It's not uncommon to have 3-5' waves in a heavy blow if the wind is north or south. Feel free to shoot back any questions about the lake you might have.
If you do make it up, be sure to sail north and say hello to the fine folks of the Neenah-Nodaway Yacht Club! We're very active on the lake so you'll likely see a few of us out.