I f you sail with Fred Olsen and are prepared to share a cabin you can get it at the standard rate without single supplement, plus you may be lucky and get a twin cabin to yourself.
Hope this help.
You are probably best to travel with one of the cruise lines whose ships have a number of single cabins eg SAGA, Fred.Olsen,P&O. I am afraid to say (as a single traveller) that you are unlikey to get the bargains for single cabins that couples can often obtain. I am afraid most ships, like hotels, are built woth couples in mind.