why arent the captains of the ship called water drivers

Dear Student

You need to understand that sometimes it's necessary to maintain a standard in our vocabulary. Similarly, language under-goes few modifications defining a certain term which maintains the standardization of a particular word. 

For example : Pilot cannot be called Air drivers. The word 'Pilot' itself means that it's the person who flies an airplane. Hence, the person who rides a boat or a ship is called a Captain.

I hope this clears your doubt.

Thanks and Regards