How can I attack a ship? I'm trying to attack a roaming army for Ardent Mages quest (defend a city and defeat 3 armies), and it got to the ocean, and now the second (!) winter came while I follow it with my army and try to attack it (and nothing happens, which makes me think that ships are indestructible, which would suck).

Fairly certain that you can't attack ships, and that embarked enemies tend to stay in boats. You just have to wait out the next summer, when they will probably land.