Most of the major cruise lines -- both mainstream and luxury -- offer Alaska cruises now, as well as smaller lines that may offer more special-interest cruises and off-the-beaten path port calls. Just as the ship you choose in the Caribbean will determine whether you stop at a small island like St. Barths or a large one like St. Thomas, the size of your Alaska cruise ship will often determine how close you get to glaciers and wildlife. But more important, large-ship cruises usually begin in one of the larger ports such as Whittier or Vancouver, sailing either one-way or round-trip; smaller ships usually stay within Alaska, beginning and ending their cruise at an Alaska port like Juneau. You'll have more access to kayaks and zodiaks on a small ship, while a big ship may offer only large-scale shore excursions.