Some 20 % of the accommodation will be for solo travellers, with a choice of single cabin grades. (silvertraveladvisor.com)
When you make a booking, you can choose from a guaranteed single cabin, which can be inside or outside. (friendshiptravel.com)
Norwegian Cruise Line is the brand most commonly mentioned for having actually built single cabins and apparently there is a dedicated area for solo travelers! (theboutiqueadventurer.com)