For European ferries, try this website which is accessible in English, German, French, Spanish, and Italian. The Ferrylines site will help you with routing and hotels; provide information about timezones, weather, and currency conversion; and book your trip for you at no extra charge.
David's photo of the Seine River - Paris
If your next trip involves Australia, New Zealand, the United Kingdom, Canada, the United States or the Caribbean, you may want to peruse this site Arthur Frommer recently mentioned on his blog. While you cannot purchase tickets on the site, you can gain valuable information that has not been consolidated elsewhere. And if you grow weary of pondering the 750+ ferry routes, you can quickly find a diversion by clicking on the blog portion of the site.