Here's what I know. Sea Miles offers either a Mastercard or Visa. No annual fee for either of them. Your earn 1 "mile" per doller spent and 2 for every dollar spent with Carnival. The reward levels they advertise are minimum requirements (i.e. free 7-day cruises may start at 50K or whatever, but the more desirable cruises are more likely over 100K miles per person). Their website has a pretty good search feature for finding cruise information. Discounts on cruises seem to be from brochure rates. You can get a rebate on airfare $25 / 5000 miles, and they have resort discounts but you have to call the toll-free number and kind of fish for resorts.
My wife & I had separate accounts until we found out we couldn't combine them for rewards, each would have to book at single rate. So we drained one account on airline rebate and added a second cardholder on one card.
Summary: be prepared to rack up a pretty large pile of miles before going on a nice 7-day cruise for two. We're still pursuing that.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013