Yes- some cruiselines have licence to marry at sea- which cruiseline you thinking of going on? I was married recently on the sea princess at sea and would be happy to answer your questions but need more details.
....what ship......what cruise......
You can get married on Princess ships. They have a wedding chapel and arrange special 'packages' for those wanting to tie the knot.
We haven't decided yet which cruiseline, but it will probably be in 2010 and sailing from Portsmouth, England [fear of flying].