Longtime solo cruiser here and, oh my yes, I'm with the folks that hate the single supplements. (Death to single supplements!) And, by the way, if I have to pay for two people in the room even though there's only one of me, I damn well better get TWO chocolates on my pillow.
As for the dining alone thing: oh hell yes. On land, on sea, in a house, with a mouse ... If I want to eat solo in the nice restaurant, I'll eat solo in the nice restaurant. Reservation for one. I'll read a book on my phone, or people watch, or just sit and think. (And once, at one very lovely tea establishment, I was even offered a selection of reading materials to accompany my scones.) Don't let them make you feel like you don't belong. ESPECIALLY on a cruise, where you have literally already paid for a table in that nice restaurant (and with the single supplement, you probably already paid for a table for two!)