This is mainly a wish list, put also part "likely add" list - that is, why each band might have been asked and/or might have accepted and/or why they would add major awesomeness to an already majorly awesome, epic line-up.
In order by most likely to least likely to be added....
1. MOTORHEAD: because they are cool, old-school, crowd favorites plus Lemmy and the boys seem like they're always up for a rip-roarin', ass-kickin' good time (plus DragoLordJones has been prayin' for this big time - and I think it's workin'!

)
2. MORBID ANGEL: because the cruise needs more death metal, they are SO important to the history of death metal, AND they are from Florida which would make them and my next pick the perfect last 2 announcements.
3. KAMELOT: because of the Epica (S.S.) connection - I would bet Epica and Kamelot were booked as a sort of natural package deal, plus....they have a new album to promote, they are a crowd favorite (certainly at Wacken 2010), and they are (mostly) from Florida. Last but not least, Kamelot doing The Haunting featuring Simone would be amazing!
http://www.youtube.com/watch?v=IWDALIOzXNk4. IMMORTAL: because the cruise needs more black metal (especially of the Norwegian variety

), they just had a successful run of five summer festival gigs and the cruise strikes me as the kind of gig they might dig, kvlt appeal, overall awesomeness
5. SCORPIONS &
6. MICHAEL SCHENKER GROUP because with Uli Jon Roth already on board, they're a natural "package deal", both their separate and potentially combined performances would be LEGENDARY, and last but not least, their announcement would convince older, more "monied" rockers/metalheads to spend the extra buckaroos on the expensive cabins that are left.
7. SATYRICON &
8. 1349 because the cruise needs more black metal (especially of the Norwegian persuasion), Satyr and Frost are very personable guys who I think would dig this kinda thing, plus Frost is the drummer for both bands - great package deal.
9. OPETH: because the cruise needs more death metal, Akerfeldt is a legendary, well-respected 20 year metal veteran, major fan base, headliner status, perhaps would like to extend his vacation in the Caribbean with wife and kids

(plus Opeth is my favorite band, hands-down

)
10. DIMMU BORGIR: because the cruise needs more black metal, they've got a new album to promote, the idea of a cruise and perhaps an extended trip in the Caribbean after an extensive European & U.S. tour may have sounded appealing to Shagrath & co. when asked to do the cruise, Shagrath & Silenoz rule
Honorable mention:
TURISAS: because these folk/viking/pagan metal folk seem to travel in packs

and Turisas is the perfect band to complete the cruise pack imo! (think about all those heathen and pagan fests many of us have been to); big fan base (certainly on this forum!), incredible live performers.