And there is absolutely no way of even knowing if doing some "super scan" would work. That is ridiculous. Especially considering his body has been altered by a Devil Fruit, who's powers still aren't clearly defined. He won't just get an answer by X-Raying Kizaru.
I hate it when people assume that a character will just automatically realize the weakness of another character even if they have absolutely no way of knowing information like that. Because sometimes even logical weaknesses may not be applicable tons certain character.
@nyas: Superman would need to know about that weakness. Which he doesn't. And of Kizaru saw big water coming, he would haul ass out of there. He is an experienced Admiral, no way he would get caught in water that easily.