Let me explain a few things to you that you think you learned from movies and television, but are incorrect.
First, most crew members are from India, Ukraine, or the Philippines (Even if the ship flies a Liberian flag, there is NO chance that any crew member is Liberian and it's quite likely the vessel has never visited Liberia). The common factor in these places are that there is a lot of poverty and unemployment. Getting one of the 22 jobs on a commercial cargo vessel is like winning the lottery to them. They're not going to do anything to jeopardize that job and the shipping line won't hesitate to fire them for the smallest infraction since there are so many vying for those jobs. Further, becoming an ABS is a long and expensive process, so they have a lot vested in those jobs.
Two, despite what's shown in movies, you can't just stroll off the vessel and into some dive bar. You need a visa. The shipping lines aren't going to get you a visa for Nigeria because they don't want you getting lost or kidnapped or killed. Even if you went through the trouble of getting your own visa, it wouldn't be of any use to you because the captain holds the passports to all 22 crew members.
Three, international ports of trade have to meet specified security standards or the vessels and their cargo are not permitted into the U.S. or pretty much any civilized nation. People can't just walk in and out of an international sea port or on and off a vessel. Port Security, Customs, and the Shipping lines themselves see to that. There's too much capital invested in the entire operation to risk it on some $8 whore.
Lastly, if somehow, an ABS did go to, say Sierra Leone, on his free time (Lord only knows why someone would want to do that), that entry mark on his visa would almost certainly deny him a job on any international shipping line.
“wo, despite what’s shown in movies, you can’t just stroll off the vessel and into some dive bar.”
Who needs to leave the boat? Vendors come out to the boats while line up waiting their turn.
Seen lots of them, frequently, while on off-shore rigs. Thye tie up and sell all sorts of stuff from fresh-caught fish to ice cream. No doubt the “ladies” do the same things.