Larry asks:
Could you have become a "marketer/advertising writer" if you'd spent YOUR
childhood at sea on correspondence courses?....

Easily. Much more easily than a typical product of the US lower/high
school system could. Our university system is pretty good, however.

and...
or would you have become one
of those poor slaves hauling out someone's nasty engine from the bilges?

Like the employees at the local fast-food place whose math skills are
so pitiful that they can't make change? These people are called "high
school graduates." Same as that poor grunt hauling your oily engine or
grinding your fouled bottom for negligible compensation.

Define "success" for me and we can have an interesting discussion about
that. I'll bet my definition differs greatly from yours. For instance,
mine includes "fun" as one of the more important components.