I'm playing a game at the moment as the English on an Arch map. I'm the greatest power, but I have the Spanish and Byzantines nibbling at my heals. So in that sort of game, navies are really good. I've developed a taste for privateers so I can stymie the AI's naval growth and freely scout out coastlines. The man-of-war also allows me to expand my navy with very little expense. Sure, it's only a wooden navy, but they still put up a good fight against ironclads (and when the AI has ironclads, I'm usually building destroyers, anyway). Nuke subs carrying tactical nukes aren't too bad either.