I suggest you tell the Maitre D' that you are traveling solo and would like to be seated with (whoever you want to be seated with--- but I assume) other non-married people cruising - whether they are sailing solo, or perhaps you could say "any ladies dining alone" or something like that.
A tip will help.
Or - if you are walking through the open dining room and you happen to see a couple of ladies traveling alone (or one) you might ask politely if she minds if you join her.
Don't get discouraged if she says no. They are a number of reasons why that could happen and cruising is not always for people looking to meet new people.