The single supplement is what puts most singles off cruising. I've done two Caribbean cruises and a Middle East cruise on my own and would book another if I could get one and not be ripped off by supplements.
One cruise company wanted to charge me noticeably more for one person in a double cabin that for two people in the cabin, even though I would have just one seat on the flight to the Caribbean. I said to them: "Why don't you put a sign on your brochures. No single people or dogs allowed."
Advice: It's always worth paying extra to get an outside cabin, and one not down in the bilges.