If you are getting married on a cruise, then the wedding must take place in a port of call. The marriage must be registered in a specific jurisdiction, so you can't be married at sea in international waters.
The only exception to this rule is the Province of British Columbia Canada - there you can be married at sea as long as you are with in the territorial waters. If you are sailing on an Alaskan Cruise out of Vancouver BC Canada, that would be one option.
here is Carnival's website - http://www.carnival.com/CMS/Static_T...s/shipwed.aspx