Dock in seaport meaning is 16th C (OED), so it is old enough. "Doxy" has been around long enough to attract variant meanings. The OED says it is slang for a mistress, paramour or prostitute, but also recognizes the dialectical meaning of a wench, or sweetheart. The OED mentions that its origin is unknown but it is "possibly a derivative of dock."
1562: "If she be his harlot, she is called hys Doxy."
1827: "Spending all my money among doxies..."
1825: "a sweetheart, but not in the equivocal sense ...Shakespeare."