He has the Sword of Triton that can control the ship, right? That's how he tied up his crew and stuff. So why does he need them anyway if he can control the ship with his sword alone?
Common reasons he would need a crew:
I take from the wikia. For point one.
Anyone brandishing the sword would rule the ship, controlling the vessel to do his indomitable will. Despite its great power, the sword's power was diminished away from any ship.
So technically he needed someone to help outside the ship, and possibly if the ship was overrun. So if you take that to be canon, then there is a answer.
He could possibly keep people around so that he is not alone. So that he has some level of companionship.
He has a wider scope with a crew
He has his sword, but the sword isn't omnipotent. He can get more things done by using a crew and if his sword fails(unlikely) he has his crew. But if his crew fails, he has his sword. Plus, he can probably get more things done, with both the crews and the sword's combined efforts.
He could possibly keep a crew, because that is what captains do. He might do it to keep up appearances and because he is a captain. So he needs people to command.
This is a very weak option though, so I put more stock in the other ones.
I don't think it was ever addressed(why he would need a crew?) in the canon. Though, I may be wrong.
Well, he probably wants to do things other than constantly concentrating on getting the ship to sail itself. Like eating, sleeping, pissing, doing anything else.
Also the sword is only useful on board a ship, so unless he plans on tracking the fountain on his own he will need a crew. He probably didn't want to carry the mermaid all the way inland himself.
Well use of the sword requires him being always at a position to use. So if he falls asleep, passes out, gets sick or everything else that could render him temporarily incapable of using the sword then the ship would follow a random course. While keeping a crew you can run a ship even while you are asleep