(FORTUNE Magazine) – DRIVERS stuck in rush hour gridlock long for alternatives. Surprisingly often the gridlock is on a bridge affording enchanting views of boats playing in the water below. Sometimes, as in the comic books, a new idea explodes in the driver's head: ''Hey, why can't I do it that way?'' Increasingly you can. In Manhattan a flotilla of new ferryboats can speed you to LaGuardia airport or drop you at six other locations in New Jersey or on Long Island. The state of Washington is adding three new Seattle ferries by 1989. The number of ferryboats serving Boston has grown from two to 13 since the late 1960s. The choicest delight for occasional visitors to Beantown is the Airport Water Shuttle, which takes passengers from the financial district to Logan Airport in 15 minutes, bypassing the chronically clogged Callahan and Sumner Tunnels. America's once-thriving ferry industry withered and nearly died when bridges and tunnels siphoned off commuters after World War II. The bridges turned into bottlenecks, and now city officials are encouraging, and in many cases subsidizing, these new ferries. Anyone who doubts the ferryboat business can be raised from Davy Jones's locker might take a run, for instance, on the Port Imperial, a high-speed pedestrian ferry owned and operated by Arcorp Properties of Weehawken, New Jersey. The Port Imperial runs from Weehawken to Wall Street. One recent rush hour on a golden September morn, the view of Manhattan glistening to port and a salt breeze wafting over the bow did more to quicken the blood than a dozen cups of coffee. The ferry, traveling at a brisk 22 knots (about 25 mph), makes the five-mile trip down the Hudson in about 20 minutes and costs about $10 round trip. A car that travels at rush hour will cost twice as much when tolls, gas, parking, and tips are included. And the car will take at least twice as long. The Wall Street service, which began in August, now carries 400 passengers a day.