at least in the US Navy:
Ensign-->Lt(j.g.)-->Lt-->Lt Cdr-->Cdr-->Capt-->Adm (4 varieties)
However, the CO (Commanding Officer) of any ship is always referred to as the Captain of the ship.
Army Captain is pretty low (same as Lt IIRC; Navy Captain = Army Colonel); this can lead to interesting situations when the services mix.
Tony