In the UK Prostitutes are knows as Pros.
Hence Pros from Dover may originate from this fact, Dover is a port
and where there were sailors there did you
also find Pros - lots of pros.
I vote for that one. It makes a lot of sense in a "folk etymology" kind of way. This kind of like playing linguishtiks, isn't it? (Anyway, that's what we called it in grad school.)