You still use left and right on board, but referring to other things. E.g. you sit on the left side of the table, and by using "left" it's clear that your refer

You still use left and right on board, but referring to other things. E.g. you sit on the left side of the table, and by using "left" it's clear that your reference is the table itself and not the ship. What you achieve is a new set of coordinates relative to the vessel. You are going north, sitting on the port, with a glass of wine in your right hand, instead of going up and sitting on the left.