The harbormaster's segments indicate that humans are present in this world, yet no others are seen and they never seem to be present in the stories. All of the boats appear to be fully autonomous and able to work independently without the need for a human crew, but on multiple occasions the harbormaster has talked about being a crew member on various boats. There is also the curious detail that on the rare occasion he interacts with one of the tugs (usually shouting to Theodore, who is always offscreen) only a whistle is heard, never their voice. There are also details like Hank having a lifeboat despite a conspicuous lack of human crew members who would need it.
He is also able to recount in surprising detail events he was not present for, down to the specific thoughts running through the minds of the boats involved.
These are very curious details which lead to one plausible conclusion- the various characters of the big harbor aren't actually sentient machines. The stories he is telling are based on actual things that occurred in the harbor, but it is obviously meant to be assumed that he is telling the story to children- who might have trouble understanding all the intricacies of and inner workings of the harbor. Plus there would be lots of people to keep track of and no shortage of confusing nautical terminology.
So instead the harbormaster simplifies the stories to make them easier to follow by turning the machines into composite characters. The various crew members of each tugboat are therefore combined and represented by the tugboat itself.
The tugs are each named after their respective Captains. This is also why Foduk has an unusual name that stands out more- the other four had easy to remember first names (Theodore, George, Emily, and Hank). Foduk's captain has an unusual first name that the Harbormaster was worried kids might have trouble pronouncing or remembering, so he decided to use the man's surname instead. Aspects of the different captains' personalities are also incorporated into the tugs.
The same is true for other boats in the harbor, all of which act as composites of their individual crew members to make the story easier to follow, especially since some of them would require even bigger crews than the tugboats.
This also explains how the tugs are often able to perform certain tasks on their own without hands. For instance on several occasions they're described being able to tie ropes and load/offload cargo on their own. It's actually the crew that's doing those things, but it's easier to just say "Theodore tied his rope" than to go into a detailed description of the different types of knots used by the crew.