I am applying for Chef work on cruise lines too (we should compare notes). I've read lots of cruiseline and hiring agency websites and a few suggest that you be min. 21-25 y/o so you might want to downplay that. I don't meen to sound cynical at all but how did you get to be "fully qualified Chef" at 18? My experience is that it takes at least several years as a full time professional plus culinary school basic training. Again, from all the websites I've read for culinary positions onboard, they ask an applicant to have at least two years continuous hotel cooking experience and basic culinary school training.
Good luck and please give me shout, I'm sure we can make it easier for each other by comparing notes. My hotel kitchen closes mid January for remodel, I've got several offers in local kitchens but I'm holding out for cruise ships.