Cruise lines quote their prices on a per person basis, based on double occupancy. So if you want to cruise solo and have a stateroom to yourself, you normally have to pay a surcharge. This is understandable, but annoying for the person who wants to take a cruise alone.
Three cruise lines have special single occupancy cabins for solo cruises. The huge Norwegian Epic cruise ship has "studio" cabins designed and priced for a solo-cruise (no single supplement surcharge). The cabins are arranged in a special key-card access area and each have a window to the corridor. There is a shared private lounge.
The new Norwegian Breakaway cruise ship also has studio cabins for solo travelers. They have the same modern style and features as those on the Norwegian Epic. The Breakaway cruises out of New York.
The luxurious Cunard Line now offers single staterooms on the Queen Elizabeth cruise ship. There are 9 cabins reserved for singles. 8 are ocean view and 1 is inside. They are centrally located midship on Deck 2 close to the Royal Court Theatre and the Queens Room. 24-hour room service and nightly turndown service with pillow chocolate among other amenities.
American Cruise Lines has some single occupancy staterooms for solo travelers on its ships, including the Queen of the Mississippi. It is a paddlewheel ship that offers cruises on the Mississippi River.