Aircraft carriers and cruise ships sail on blue oceans with immense reserves of power and degrees of freedom. One sort of ocean-going vessel is devoted to aggression and war, the other to comfort and leisure. Both, however, are massive, independent, floating islands of power. They navigate "strategically"—based on large-scale charts, covering vast distances, driven by economic or military considerations.
Tugboats, on the other hand, are limited to a specific harbor. A tugboat master may be one of the highest paid individuals in the shipping industry, but only in one place. To be a tugboat is to be committed to a specific place and to know it intimately. Tugboats have to be nimble, maneuverable, and responsive to the slightest variation in the sea floor or the local currents. Tugboats are not especially impressive, mechanically or visually. But they are indispensable. Tugboats, you might say, are servants. They don't navigate for themselves—they navigate to bring other ships safely to shore.
