Bridge Bay Marina runs hour-long Scenicruiser boat tours of Yellowstone Lake  ($14 adults, $9 ages 3-11) several times a day in the summer, providing a fine introduction to the area.
Also available at the marina are guided fishing charters (starting at $152 for two hours for up to six people) and boat rentals ($10 per hour for a 16-foot rowboat; $47 per hour for an 18-foot motorboat with a 40-horsepower outboard). Fly-fishing guides are also available at $418 for two people per day.
The best Yellowstone Lake fishing is from boats rather than from the shoreline. If you’re bringing your own boat or canoe to Yellowstone , pick up a park boat permit—and check out the regulations—at the Lake Ranger Station or Grant Village Visitor Center. Boat slips are available at Bridge Bay Marina.
All streams are closed to watercraft, with the exception of the Lewis River Channel between Shoshone and Lewis Lakes. Motorboats are allowed only on Lewis Lake and parts of Yellowstone Lake.
Several companies offer guided sea kayak trips in the park. Snake River Kayak & Canoe School (307/733-9999 or 800/529-2501, www.snakeriverkayak.com ) leads a variety of trips in Yellowstone, starting with three-hour sunset paddles ($125 per person) on Yellowstone Lake and half-day paddles in the West Thumb  area ($175), all the way up to six-day trips that leave the crowds behind as you explore the lake; these cost $1,250 per person, all-inclusive. The company also guides sea kayaking tours of Lewis and Shoshone Lakes and has a full range of kayaking classes in Jackson  and Grand Teton National Park .
Jackson Hole Kayak School (307/733-2471, www.jacksonholekayak.com ) has day tours of Yellowstone Lake or Lewis Lake for $195-295 per person (rate depends upon the number of fellow kayakers). They also offer multi-day treks, including three-day tours of Yellowstone Lake or Lewis and Shoshone Lakes for $875 per person. Multi-night trip prices include transportation from Jackson, kayaking and camping gear, supplies, and all meals.
O.A.R.S. (209/736-4677 or 800/346-6277, www.oars.com ) offers half-day sea kayak trips from Grant Village  for $95 adults or $70 kids. These can typically be booked the day before. In addition, they guide multi-day trips in the Yellowstone-Grand Teton region, including a six-day paddle, float, and hike adventure for $1,540 adults or $1,385 kids. Tents are included, and sleeping bags are available for rent if you lack one.