I think it's worth having a discussion about the H.L. Hunley and whether it belongs on this list. The two issues I can think of being whether you want to consider it a successful submarine in spite of never fully submerging during combat. The second being whether you want to dignify the Confederate States of America by listing them here among other independent states. I would be inclined to list it with an asterisk or two.
Absolutely, I think it's worth mentioning, even if I do see QuizFace's point on the technicality of it being a rebellion as opposed to an internationally recognised country. Regardless, although I'm using the Wikipedia list as my source, I'm not particularly happy with it, as I did a bit of digging and found a few countries-that-were which absolutely operated submarines and yet aren't listed!
I think I've kept it alphabetical just so that folks can get some of the more obscure answers, but I take your point! It surprised me too how many operators there have been!
Neat quiz, I had no idea they were so prevalent.