Cruising is ideal for a single person. I will be going on my eigth cruise, this year, all of which have been as a singleton, with single occupancy of a twin cabin, and all with P&O. It is very easy to meet new people on board, you can join in as many activiites as you want, or quietly sit and read, there is no pressure to join in, if you don't want to. One tip I would give is when you make your dining request, ask for a large table, six or more. You meet far more people this way.
Probably one of the best kind of holiday for singles. It can prove a little expensive for single travellers for, like hotels, they cater mainly for couples. Several cruise lines eg SAGA, Fred. Olsen, P&O have dedicated single cabins but on others you can book (at a price)a double cabin for single occupancy. On board ship it is easy to integrate into the community and make friends. You will not be the only "singleton".