Several companies lead guided tours of Portsmouth Harbor, taking in the islands, lighthouses, and scenic waterfront. The most intimate is the Tug Alley Too (47 Bow St., 603/430-9556 or 800/884-2553, www.tugboatalley.com , May–Oct., $49 adults, $29 children under 12), a tugboat that once carried lobsters to islands on the Maine  coast. Now owners Bob and Natalie Hassold take her out for one-and-a-half-hour spins around the harbor, with a maximum of six passengers on board.
A cheaper but more crowded tour is offered by Portsmouth Harbor Cruises (64 Ceres St., Portsmouth, 603/436-8084, www.portsmouthharbor.com , $16 adults, $14 seniors, $11 children), with several different routes aboard the M/V Heritage.
You can get out on the water in decidedly smaller craft through Portsmouth Kayak Adventures (185 Wentworth Rd., Portsmouth, 603/559-1000, www.portsmouthkayak.com ), which leads guided trips around the Piscataqua River estuaries as well as out on the open ocean of Portsmouth Harbor. A two-and-a-half-hour trip is $59. Full-day rentals are available for the same price.