I played on a continents map the other day, and i found the navy very useful indeed. Japan had lots of large coastal cities, including the capital, and my armies were too busy attacking other cities to the south. I used my large navy to pillage their sea resources and to take down the city defences. By the time my army was able to reach the capital, the navy had already done much of the groundwork, speeding my progress throughout the Japanese Empire. I only wish the navy had more of a role; for example, attacking trade routes (a.k.a Call to Power). This would add a very interesting game dynamic; perhaps incorporating convoys somehow. You don't necessarily have to see the convoy ships, you could just attack a trade route line, and steal commerce/resources every turn until the enemy is obliged to do something about it; building their own navy, perhaps even a deicated submarine force.