I made this enquiry last year as I was thinking about getting married on a cruise too! The following cruiselines have wedding licenses & offer great wedding/honeymoon packages: Carnival, Celebrity, Island, P&O Cruises & Princess. Good luck.
If you look on Princess cruises web site you can look at the web cam on all their ships to see the wedding chapels on board. If you are lucky you may be able to see a wedding taking place on one of the ships. I did take a look at the chapel while we were on board Coral Princess in Feb this year and it looked lovely. On our cruise there were 4 wedding.