Well, any of the civs are potentially situational. England, for instance, is better on bigger maps. A small map doesn't give them much, due to the extra spy not having as many options and the raw number of turns your ships gain from moving around a bigger map.
Korea isn't as good when ocean separates civs, since they are slower to the ocean than anyone else and not meeting people until a tech later can potentially slow down some RA's and prevent them from getting a few techs faster simply due to not knowing civs when they go back to fill in techs after a beeline. On the other hand, that UU could end the game, period, if everyone is connected by coast.
Tiny islands doesn't give the Ottoman UU's a chance to shine, reducing them to a UA (a good UA for the situation, but still...), while bigger islands allow their big navy and strong military to both take hold of the game.
Strangely enough, I'd say Japan deserves on the list. Though their UA isn't near as good as it used to be, in a naval invasion, it can make a big difference. Ships can't heal while out at war without a promotion or taking a city, so having them fight at full strength even after they get dinged up can help keep the push going (and it's one of the few situations when the Zero looks halfway decent).
I'd probably give Etheopia a spot too. Everyone (except Polynesia) is going to be restricted to fewer cities, so getting more faith from those cities is huge to a religion and land battles often happen near cities and the capital is normally a big more exposed, since there isn't much land outside of cities and capitals are normally coastal, making the UU ever so slightly more useful.