Aqua, not only is Wikipedia wrong - though it does say it's only a claim - about 'landward' rather than 'northern', but 'posh' never did have anything whatever to do with boats!
The very first time the word appeared in print, meaning 'grand/swell' was in 1918, having earlier appeared as 'push' - with a "u" - in a P G Wodehouse story in 1903.
As British soldiers, officials and officers with wives and families had been sailing to and fro India for almost three centuries by then, it's clearly too late for the �port out' explanation to have any substance.
It was also rejected in the 'Mariners' Mirror' decades ago and presumably sailors of all people would have known. Finally, the steamship company concerned, P & O themselves, deny the phrase ever even existed!
According to The Oxford English Dictionary, it is probably no more than a corruption of Wodehouse's 'push'. It goes on to say the legend (quote) "lacks foundation".