The ship (the Titanic) is not being described as prepared. The line makes logical sense when it is taken with the last line of the previous stanza, where there is no ending punctuation after "everything" indicating the thought continues.
"The Immanent Will that stirs and urges everything
Prepared a sinister mate"
"Prepared" isn't an adjective describing the Titanic. It's a verb describing the actions of the "Immanent Will." You can think of the Immanent Will like a supernatural being. Fate, God, etc. Regardless, most of the second half of the poem is about the construction of the Titanic and the iceberg. That's the cool part about this poem. It presents the Titanic and the iceberg as two sides of the same coin. One cannot exist without the other. They are intricately tied together, and they always have been. As men were building the perfect ship, a higher power was building the perfect slab of ice. The word "prepared" helps deliver that idea. "Prepared" carries ideas of intent and purpose. It's not a random "Shape of Ice," but designed.