What are the yachts in the poem by William Carlos Williams? How does this fit with Williams's dictum "no ideas but in things"?

Williams's precept "No ideas but in things" is found in a line from the 1927 version of his poem "Paterson" and encapsulates his Imagist approach to poetry writing. Williams believed that poetry should focus on objects rather than concepts and that it should, essentially, move away from the abstract characterization of things. Objects or things create visual images and can be tangible or observable. The image that one has of a thing creates an idea about it and, therefore, gives it meaning. Thus, ideas exist in things.
In this poem, Williams describes the yachts in great detail. He provides the reader with carefully constructed images of their movements, what they look like, and those who man them. They are delicate objects, and the crews operating them seem almost insignificant; they are described as "ant-like." This confirms the fact that they (the yachts) are the central focus of the poem.
Smaller crafts follow in the yachts' wake, trailing behind and following a majestic object. They are like minions escorting a supreme leader or king. They follow in admiration, ready to do its bidding. 
The pleasant descriptions of these objects soon change, though, when the focus shifts from the yachts to the ocean. The last three stanzas convey a grim and bleak picture and personify the ocean as a mass of bodies despairingly clutching and reaching out to the yachts. The vessels, however, are not disturbed and "skillfully" continue their journey, ignoring the desperate cries of the sea, which has become "an entanglement of watery bodies."
It should be obvious that the objects in the poem depict specific ideas and are representations of more than what they seem. The yachts signify a great power or presence that is uncaring and reckless. It uses the ocean to meet a goal and does not consider the object (the ocean) it utilizes to further its end. Many interpretations suggest that the yachts symbolize the privileged elite who exploit the poor in order to enrich themselves and maintain their status.
It has also been suggested that the yachts are symbols for slave ships, on which the most horrifying atrocities were committed. The sea, becoming "an entanglement of watery bodies," represents slaves who were thrown overboard, either dead or alive, during the horrendous journeys undertaken by these vessels.