But remember: People mentioned "Radical Dreamers" as causing havoc on the mainland. Not to mention the fact the Kid's first known locale was Lucca's orphanage (it's possible she was found in a forest in El Nido, but it's more likely Lucca found her close to home). It's possible that after the events of CC, when the timeline reverted to an ideal state, Kid was placed back in the mainland, where Schala had originally sent her. Also, how do we know the boat is of El Nido design originally? We know what Porre's military boats look like, but their civilians may use boats that look like the one we see Kid riding. It might not even be of Porre, but of some other nation (in one of Norris's quotes, he makes it clear Porre is fighting with some other nations). It's even possible that someone rode that boat from El Nido to the mainland, at which point Kid stole...ahem, bought it.
My point is that there are far too many unknowns for us to say for sure one way or another what Kid is up to. Which brings us back to the possibility that the producers left it vague for a reason: so we can decide for ourselves whether Kid is coming to meet Serge, or going off on her own.