This was in part inspired by some jokes about Shanks having the Swim Swim Fruit, so he didn’t drown because of that. But that made me think, what if Shanks in fact had a Devil Fruit power back then, but he is so massively strong and has such a strong will (Haki), that he was able to keep afloat for a bit?
Think about it. Shanks got to Luffy in a split second all the way from the shore, so he is super fast, but after being in the water he did nothing but move slowly and intimidate the Coast King, he intimidated him, he didn’t knock him out with a kick or use some other move, he just stared at it, in fact after rescuing Luffy he didn’t move from the spot, only glared the beast and raised his hand to put his hat on Luffy, if he could do that, why could he not take his sword and kill the Coast King, it would only take him a split second to do so.
Well maybe that was all he could do, because he was using all his strenght/Haki to just keep floating there, because he was being drained of all his power by being submerged.
Additional information: after rescuing Luffy, it is never shown Shanks swimming back to the shore, he didn’t attack anything, you tell me you were capable of reaching a kid in less than a second that was hundreds of meters from the shore, but then when you reached it, you couldn’t swim back, even a little.
Most would say that the scene ends there for dramatic purposes but practically speaking if you were trying to save a kid that can’t swim from drowining, would you stay with him in the middle of the sea, or would bring him to safety before talking to him about what being a pirate is?
*Theory by With-a-Don