Sure, but you have to start somewhere. If people can't deal with operating a cruise ship then I question their ability to create brand new floating islands and run them sustainably. Surely one could do some farming on a cruise ship (using a mix of decks and hydroponics) and likewise generate some energy on-board. Obviously not as attractive as running a small nuclear power plant but you probably can't buy a used nuclear aircraft carrier, so you're stuck with diesel in the short term.