My grandfather was ship’s bookie on various battleships during the period 1916-26*, and a merchant seaman until well into the 1960s (he spent his later years teaching me how to read racing form and ridiculing my selections). It is fair to say that he had been on more ships than most during his lifetime. Being an intelligent man, he would quickly have worked out what is meant by onboarding, but I doubt he ever considered himself to have done so during his lifetime.
*of course this was not his official rank…