Serious sailors are not likely to live full time at this altitude; even weekend sailors would be wise to look elsewhere for their kind of fun. But if you insist on trying, you'll find a few sails spread on the bigger lakes of these mountains. You can take a pretty big boat on pretty big Flathead Lake, Jackson Lake, or even Yellowstone Lake if you're careful about the weather. Smaller boats such as Hobie Cats, in some ways, better suit the sudden, swirling winds typical of these mountains.
Powerboating is another matter; if you've got a motor, pack a lunch and head for any of the many lakes that dot Montana and Wyoming's landscape. Canyon Ferry is a popular Montana water-skiing spot, and you'll see Wyoming powerboats cruising Boysen Reservoir or the many impoundments on the North Platte. Just make sure to check around locally regarding access if you're uncertain about it. All types of boats are available locally for rent.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.