Actually, what I ended up doing was this:
The sailing: SS Polynesian, sailed April 20, 1882 is where my ancestors' profiles point to, because that's the particular sailing they were on.
Above that, I created a category called S.S. Polynesian, so that people working on profiles of people who were on a different sailing can link to there, too, and possibly that category will end up having all of the Polynesian's sailings listed under it.
The S.S. Polynesian category is now a subcategory of the 1870s Ships category, since the Polynesian was launched in 1872. But the S.S. Polynesian category also points to the S.S. Polynesian free space page, which has all the details I collected on the ship itself. (Apparently maritime history is a popular hobby all on its own, because digging up the details on the ship was pretty easy.)
I was tempted to put the details about the ship into the S.S. Polynesian category page, rather than having a separate free space page, but I couldn't see any way to add the photo (engraving, actually) to the category.
Greg