Guide to the best cruise ship shore excursions. Sept. 6, 2010— -- Snorkeling through coral reefs, exploring ancient Roman ruins, hiking on glaciers, shopping at local outdoor markets; all ...

Carnival aims to make it easier by offering many hosted group excursions, where a representative from the ship takes you to a place or activity where all transport, and often meals, are included.

Mason Gravley sits atop his board during his excursion across Lake Okeechobee with a friend, Jordon Wolfram, on Dec. 31. The journey took them more than 10 hours.

Until now, cruise lines have had a near monopoly on the sale of shore excursions, which make up about 25 percent of a typical cruise line's profits. Viator's service amps up the price competition.