Roy Palmer had this to say on the subject (Bushes & Briars: Folk Songs Collected by Ralph Vaughan Williams, 1983/1999):

"At the end of the 18th century, when most men wore knee-breeches, sailors (apart from officers) wore trousers, and had been doing so for some fifty years.  (Incidentally, the revolutionary French sans-culottes were so called, not because they went about with bare posteriors, but because they, too, wore trousers in preference to breeches).  A sailor could easily roll up his wide trousers when decks had to be scrubbed, or seas were breaking over them.  The trousers (usually spelled "trowsers" at the time) were often stained with the Stockholm tar used on the standing rigging, and "tarry trousers" were thus the unmistakable badge of the sailor."

Malcolm