Look no further than Europe to find cruising's current nerve center. A weak dollar mixed with a been-there, done-that feeling toward the Caribbean has turned the Old World into the new frontier for cruise enthusiasts. This is a golden opportunity for travel agents, particularly because of the variety of options at their disposal. Yes, there are the big players (Carnival and Royal Caribbean come to mind), but, of course, they can't give you Vienna or Budapest—they're a bit too voluminous to ply the Danube, the Seine or the Rhine. That's where riverboats step in. Riverboats not only fill a niche, they also are gathering looks because they offer an altogether different experience than what cruise ships offer.

Until May, the Cruise Lines International Association (CLIA), which represents 24 member lines and dedicates itself to the growth of the cruise industry, had not one river cruise company on its roster. Skip forward to July: CLIA now represents two: Uniworld Grand River Cruises and Majestic America Line.

What does this say about the landscape of the industry? Well, for one, it's changing. CLIA's president and CEO, Terry Dale, said the additions were made to "further increase the diversity of our member lines." Yes, but it also demonstrates that river cruising is becoming increasingly popular as an alternative to big-ship cruising. The reasons why are quite clear.

While ocean cruising has had its roots for some time, riverboat cruising is still in a nascent stage.

"It's a very new concept to the American market," says Rakesh Dewan, riverboat product manager for Tauck World Discovery, which looks to leverage its experience in the land-tour business to promote its river cruising division. Its new luxury riverboat, Swiss Sapphire, debuts next spring as a sister ship to Swiss Emerald, which launched last year.

A break for river cruising came in 1992 with the opening of the Main-Danube Canal, which connects the Rhine to the Danube, and provides 2,200 miles of unobstructed waterways running through 15 countries. With it, a boat could sail from Amsterdam all the way to the Black Sea.

"Before that," says Rudi Schreiner, owner and president of Amadeus Waterways, "you only had shorter-distance cruises."

Schreiner is often referred to as one of the early architects of river cruising. He was vice president of product development at Uniworld, followed by stints with Viking River Cruises and Avalon Waterways. Two new ships, Amacello and Amadante, will join Amadeus next year, with more ships on order for 2009.