First you need to decide when to take your tour. A cruise at the beginning of your visit is an interesting way to get oriented, but if you wait until the end of your trip you'll get to see all the sites you've seen on your trip from a totally different perspective.
And do you want to go in the daytime or at night? At night some of the boats flash big searchlights on the monuments as they sail past, and you also get the added thrill of seeing the Eiffel Tower sparkle with lights during the tour.
What type of tour would you like? Do you just want to see the sights or would you like to eat a meal? Do you want a romantic setting where you can propose marriage, or are you comfortable rubbing shoulders with hundreds of photo-snapping tourists, chattering away in dozens of different languages?
Most cruises last one hour and go up one side of the Seine and down the other, turning around at the end of the Ile St.- Louis and at the Statue of Liberty replica at the Pont de Grenelle. Most offer live or pre-recorded commentary during the cruise.
The Bateaux-Mouches cruise starts by the Pont de l'Alma (Alma Bridge). It's the most popular of the cruise lines and includes commentaries in seven languages.
The Bateaux Parisiens starts at the port de la Bourdonnais, near the Eiffel Tower. Commentary, with musical accompaniment, is provided in French, Spanish and English. Romantic dinner cruises with orchestra are available.
The Vedettes de Pont-Neuf cruise starts at the Square du Vert Gallant at the western end of the Ile de la Cité.
The Vedettes de Paris cruise starts at the Ports de Suffrin, but its tour commentary is offered only in French.
The Batobus Tour Eiffel serves as both a tour boat and water taxi, starting at the Eiffel Tower and stopping at the Musée d'Orsay, St.-Germain-des-Prés, the Cathedral of Notre-Dame, the Jardin des Plantes, the Hotel-de-Ville, the Louvre and the Pont Alexandre III.
The three-hour Paris Canal cruise starts at either the Musée d'Orsay or the Cité des Sciences at Parc de la Villette, going up and down the Seine and the Canal St.-Martin
The Marina de Bercy cruise starts at Port de Bercy, with two-hour trips for lunch and dinner.