I read in one of these responses that Holland America was a good bet but most do charge 50/70% more for a single person very unfair.
YOU'LL BE LUCKY! These sort of offers are very few and far between, and not without strings when they do appear. The cruise line would be verging on desperate in order to offer such a deal. The itinerary, the ship, or both would probably be dire for a start. The cruise lines really hate doing this as the simple fact is that 2 people in a cabin paying half price will spend a lot more onboard than 1 person paying full price in the same cabin! Be wary of promotions where "No single supplements" is another way of saying "No discounts"! No single supplement on the brochure price would mean that you are probably paying about twice as much as you could for that cruise! Happy cruising.
I agree. As a person who cruises as a single every time, I can only say that I have been lucky enough to have "no single supplement" on only one occassion, and for this I needed to book 18 months in advance, i.e. when the first bookings are taken. I can say that any time after this, it is highly unlikely that any cruise line will make a "no single supplements" offer, usually the single supplement increases the closer to its gets to departure date. I may be wrong but wish you luck in your search for "no single supplements".
Why not try to book on a ship with a number of single cabins? You will pay probably less than you would for a double cabin for single occupancy. Very occasionally you will get some very cheap offers for single cabins. Try Fred. Olsen, SAGA, P&O etc.