Meh, who cares about navy anyway?
1. Blockade
2. Avoid economic loss by blockade (needed for troop building)
3. Sea bombardment (Ships are more mobile than siege weapons)
4. Stop invasions of land (it's easier to kill a transport at sea than four infantry on land)
5. Attack far flung empires with a war of attrition (troops can't be everywhere)
6. They're easier to move from isolated islands than troops
Navies are important for support. If you attack your enemy at sea, they'll never reach the shore. Maybe it won't win the game by itself, but having a naval advantage can be a decisive advantage.