Sadly it turns out that there is not reliable evidence for this derivation of the word POSH, but it’s so nice I’d finished the sketchplanation before I knew it. The story went that if you were travelling by boat from Western Europe to India and the Far East by boat back in the day the rich people naturally took the best cabins. And the best cabins on the way Out, were on the Port side so you got a great view of the coast all the way around Africa. And on the way back on the Starboard site to enjoy the view once more. Et voila POSH as a synonym for rich. After much digging I had to conclude that the story was apocryphal, but it’s nice nonetheless.
I apologise for maintaining this rumour.