One of the things about Star Trek is everyone refers to each other informally, apart from the Captain. So it's 'Captain' this and 'Captain' that, and the reason for this presumably is because if they called him Jim it would just sound like they are all in the pub. For instance "Captain there appears to be a Velcruscian Battle Warrior on the stardboard side" doesn't sound the same as "Jim, there's a Velcrusian Battle Warrior on the starboard side. And look at the tits on it"