And remember Seastone doesn't turn off Devil Fruit powers, it makes you too exhausted to use them. Luffy under water, or luffy with Seastone on his body is still rubber and stretchy. By the way Seastone works a Seastone bullet should still bounce off of Luffy's Rubber body.
Since most Devil Fruit users are vulnerable to Regular bullets anyway Seastone ones are kinda pointless. And the ones that are immune to bullets, Seastone ones won't help there either.
Hasn't this debate happened three times already? Not to pick it up in its entirety but in short... Logia users don't dodge attacks with seastone (at least not the way they do with normal attacks). Luffy was susceptible to Smoker's seastone weapon, even though it was blunt.
Water is not seastone, albeit the effects are similar but not identical.