How? why?

Tf

Her father is some random ahhhh marine that did it with some random hot pitate or smh like that

Good points, but why would Imu go out onto a random island in the east blue and b%#g some random woman just to have an abandoned child

Absolately not.

I believe that Imu had secretly married a world noblewoman and thus Nami was born. After that, Nami's real mother had died sometime later and Imu left Nami in the Oykot Kingdom for officials from the Oykot Kingdom to take care of her.

Yea i doubt imu has any regard for anyone but herself/himself.

I believe Nami's birth was part of the Perennial Youth Operation. Only the Norona family participated in the Perennial Youth Operation.

It makes way more sense than I expected, I'll give my vote for that. And yet... we still don't know who exactly were the Nerona family, and the circumstances seem too random. I don't think this is the case.