A popular definition seems to be that P.O.S.H was printed on tickets for P&O cruise ships in the early 20th century leaving India. These tickets were for the more desirable rooms which put you on the Port side going out and the Starboard side returning home as it kept you out of the sun and kept the room cool.
However, no tickets have ever been found which had P.O.S.H on the ticket.
I suggest that the mistake has been made from the full name of the Cruise Liner: Peninsular and Oriental Steam Navigation Company. Maybe a printing error produced P.O.S.H rather than P.O.S.N.C
A more factual defintion still derives from a shipping term which does not seem to be suggested on the previous source sites.
When boats leave a harbour the shipping lanes work to the rule that you must stay to the left hand side like on U.K roads. When leaving a harbour you therefore keep the Port lights of the shipping lane on the left and the Starboard on the right. However, when returning you keep the starboard lights on the left and the Port lights on the right. Hence Port Out Starboard Home... P.O.S.H
Salcombe Harbour uses this Port Out Starboard Home or p.o.s.h. principle.