You're right, the expression is a maritime one and means that the sailor has a girlfriend everywhere he routinely sails to, each woman possibly not knowing about the others or at least turning a blind eye to them (meaning choosing not to see them). You could use this metaphorically to mean having several girlfriends at the same time. Presumably you wouldn't be able to do this if you were shy. You're also right that the phrasing in your example does sound a bit peculiar. The use of "wonderful" here hints at exclusivity rather than promiscuity. So I wonder if he means that he sees one particular girl, that he was able to woo after setting aside his shyness, as providing a more satisfying relationship than having several flings on the side. We'd probably need to see more of the context to tell.

This sentence does not make any sense to me: -- Luckily, I now have my wonderful girl in every port and can make both male and female friends equally easy now. What does having a "girl in every port" have to do with "both male and female friends"? Sounds like he is a bisexual jerk to me. Perhaps he is using "in every port" to mean "in various cities" that he travels to frequently. Frankly, your guess is as good as mine. I wouldn't fret over this phrase in this sentence. :)

Sounds like he's boasting how good he became at finding romantic/sexual relationships, and has multiple partners spread across the country/city.

Oh, a week ago I found this expression in Jack London's 'Martin Eden', in Russian version :) It was my mistake to think of this expression as a metaphore...

Все правильно))