As with other excursions, we booked this one through our cruise line so we were committed no matter the weather.
Turns out, the weather doesn't matter in a duck boat: we were nicely covered and never left the vehicle. These are different from those we've taken in places like Hot Springs: they are specially built to be "Alaska tough."
The tour host and driver have a "witty repartee" that kept us adults chuckling (not so funny for my nine-year-old daughter, though, who missed some cultural allusions; your children may, likewise, find it less humorous) while also educating us about salmon, bears and local history.
The transition from land to water was superb as we toured the harbor, seeing dozens of eagles (photo hint: raise the window to take a picture, then lower it to keep out the rain).
Ketchikan is a small town, so the tour is not really time-consuming, giving you opportunities to take other excursions or to explore town on foot.
They really do sell the "duck quackers," which you can give to folks back home, especially children. But, you might not want to hang around too long after they start quacking!