Everyone has made some very good points.
I would just like to add that if your daughter and her friend take any foreign languages, cruises are a perfect time to be with people who dont speak English!! Chances are you are going to an island thats native language is French or Spanish, and even the little things like reading the signs is way cool. My language teachers love it when i bring back pictures and cultural items (like local candy or souveniers) from the places ive visited.
I hope this helps persuade her step-father!
NCL Dawn 6/21/06 Bermuda
10/30/08 X Constellation Caribbean
Costa Fortuna 3/08 Eastern Caribbean
Costa Magica 2/07 Western Caribbean
HAL Noordam 11/06 Eastern Caribbean
X Zenith 6/06 Bermuda
HAL Zuiderdam 5/05 Western Caribbean
HAL Maasdam 6/04 Canada/New England
CCL Triumph 12/03 Western Caribbean
CCL Victory 11/02 Eastern Caribbean