The Mackinac Breeze catamaran sets sail four times a day (noon, 2 p.m., 4 p.m. and 6 p.m.), except when it doesn't. Know before you go that this activity is highly unpredictable and wholly weather-dependent. What you think is a perfect breeze might be too gusty for the day's outing, and trips can be cancelled at the last minute if there are too few participants.
If you do go out, wear soft-soled shoes and feel free to bring snacks if you want them. The boat company provides soft drinks and narration about the sites you'll pass, including Round Island Lighthouse, Bois Blanc Island, Round Island, Arch Rock and the occasional Great Lakes Freighter.