"fresh off the boat" is a phrase comes from the escape of the Vietnamese people after South Vietnam fell to the North. Most of them fled the country from the communists by boats (not ships!) to the neigboring countries as far south as to Malaysia. Because of the US had an moral obligation to them, they are then flew to the US or other farther countries by plane. Hence the original meaning is a new immigrant who does not know anything about their new home. Later, the meaning became a newbie entering into a new environment. "So you are fresh off the boat? Don't worry, after a week working here, you will become as native as us."