Some very good points so far. To add:
1. The competition in the Caribbeans is cutthroat. Pretty much every cruise line now has a presence. (You can look at the prices being charged in the Caribbeans vs elsewhere.) Unsure how much more capacity DCL would want to add in the region.
2. The Caribbeans is also the least fuel hungry of most destinations. Calmer waters and closer distances help. BUT, using a more fuel efficient ship could help with the margins.
3. Barcelona is supplying LNG to one of AIDA's new cruise ships. It's likely then that one of the new ships in Europe is home-ported in Barcelona.
http://www.greenport.com/news101/lng/cruise-ship-fuelled-by-lng-while-docked
4. LNG could be supplied through a bunkering system or even at sea:
https://www.lngworldnews.com/msc-cruises-arta-develop-new-lng-bunkering-system/
5. You can expect two ships to operate in Europe in the summer. Current DCL prices being charged in Europe pretty much seals that one. Same story for west coast/Alaska. The Alaskan prices in mid-June through August provide insane margins.
6. Panama Canal needs one of the smaller ships. Similarly, TA and east coast/Canada would also be best suited for a smaller ship due to the off-season dip in demand.