Theme cruises are already extremely popular with hundreds of them sailing every year.
For a list of them just go to Theme Cruise Finder lists over 500 Theme Cruises...
There are theme cruisers of ALL kinds, and I am sure they will only get more popular for several reasons:
(1) ships are ultra convenient for exhibition companies; they already hotel, theater and food facilities. All these facilities are in very close proximity.
(for more on this go to http://seasite.com
, the home of Landry & Kling)
(2) it is way to add value to a cruise (and hence charge more for the same cruise)
(3) It can have tax advantages for corporations, etc.