One of the many downsides of a long running series, he sacrificed Sanji's charm for purely powerscaling purposes. So that he doesn't lose relevance as one of the future Pirate King's heavy hitters.
1.Devil fruit route
2.Haki route
3.Genetic freak route
Those are the 3 avenues he could have went with, Oda went with #3. His dream is all blue, it just doesn't sit right with Sanji's character to be unable to swim. Maxing out haki would just be too soon for him, would devalue all the haki masters and even Zoro, if Sanji is somehow able to keep up at such a young age. Genetic freak it was then. I just wish it could have come without all the Vinsmoke baggage.