Have you looked into Randolph & Legman's collection of bawdy Ozark songs, mostly collected 1918-54? Not exactly comparable to shanties, but probably a similar level of taste. Many of Randolph's sources learned their material around 1900.

Most of the stuff is verbally crude and direct, more outspoken and outrageous than clever; not much is what you'd call pornographic. (There are a few stanzas of "The Hog-eye Man" that may rock you back, however.) Often the vulgarity is mixed with racism and misogyny, though I don't think it ever quite reaches the gangsta rap level.

Kind of like very crude high-school humor.

I imagine shanties ran the gamut(especially if Harlow's bawdy "Sailor's Alphabet" resembled the current rugby "Alphabet," which is not all that unlikely). My guess is that most of what was both common and unprintable by recent standards was more scatological than sexual, and, when sexual, more comic or grotesque than anything else.