Well, there are a few speculations that Marco is immortal/way older than he appears. So it could be quite possible that he learned navigation when he was a child and when he had learned everything, moved on to other fields, one of them medicine. With the Whitebeard crew being as large as it is, it makes sense that he would fill the role most needed when he has different qualifications.

touyanii:

i assume that he is much older than he looks and i even start wondering if he even might be older than oyaji was. at the very begining i actually guessed him just slightly older than ace but seeing him with shanks made me change my mind.

ace’ing a field and moving on to the next huh…

hc that marco has a graduation cap wall now.

and about the immortality.. i don’t know. that would be a really sad side effect of his powers and i wouldn’t want him to keep seeing his friends getting older and die. then i would prefer if he just aged slower than others. although that’s also kinda shitty for the same reasons. i hope he’s just smart then and studied both fields.

but since he has phoenix powers, there’s also a possibility that he gets reborn in some way. and maybe not in a immortal way. although, it would still be immortal but different. he could maybe get reborn when he dies of age (and of age only, because getting reborn after dying in fight seems way too overpowered). and maybe he could remember some of the skills he’s learnt before, and to make it angsty, NOT the people from his previous lives. like that he could also move on without the loss of his friends? idk