By default you are the Captain of your own vessel this is predominantly for display purposes and offers no immediate benefit.
If someone Info’s your vessel they will see your name and picture listed as the captain of the vessel.
This makes sense at least initially but as you level up you can get several ships and technically it would be impossible to physically be the captain of more than one ship at a time simply because a captain needs to relay instructions to his crew and how are you going to do that if you are not onboard the ship…
You can now hire a captain for any of your vessels, to do so visit a Port Master and click the Hire Captain button you should see a list of captains you can hire and the special bonuses they bring.
Once you hire a captain your ship will then show that name and picture when using the info button, however you can change it back to showing your name and picture at any time for free by hiring yourself via the same screen.
You can only have one captain on a ship at any time so if you wish to get a different captain you will lose the one you have currently got aboard the ship.
The cost and availability of these captains varies depending on the Port Master you visit and your faction!