Thanks for the ideas. Please keep them coming... there's good stuff in there that I'll use.
Overall I get the impression that people prefer to play guests (don't blame you). Therefore I would think main characters will all be guests and crew maybe used as NPCs - after clearing the use with the GMs.
Also I decided (finally) that there will be no person under the age of 18 allowed. Those characters that are younger better have good fake IDs and hell of a great story reason to be there/have gotten those fake IDs.
Also if you think about making a character think about there the money is coming from. This cruise will be expensive and although I'm not making this part of the game you might want to keep in mind to need a character that can spend in the lines of USD 10,000 to USD 30,000 a week for this kind of pleasure... So just for you to have a better understanding of who your character is, you should have an idea where the money is coming from. Might be some inheritance, daddy's credit card, bank robbery
... really I'm not very judgmental, but the regular "saved it up from flipping burgers" will probably not come up with this kind of money.
We are talking filthy rich and shameless...
Also I have started up a world building thread
for this and will get posts ready. The thread is labeled closed for now, but I will open it for suggestions once there's some meat in the posts...