Captain (N) is four bars with the Naval swirl. Why the (N) behind some ranks? Well, it is so that our Royal Canadian Air Force (RCAF) and Army members dont mistake a Captain(N) with a Colonel in their organizations. A Lieutenant (N) is equivalent to an Army or Air Force Captain. Ahhh...such is life in the Senior Service...and that term will be explained in another cache.
Captain (N) rank now. As for sea borne platforms, you could command an AOR (a tanker to supply fuel/supplies) to the fleet at sea. You may also be at sea commanding a Task Force or Task Group (a colletion of vessels operating as a coordinated activity). Other than that, you better like administative duties!
Cnegvny fghzc.