@Louis:
First, please don't mention movie crap or filler anime crap when making reasonable arguments.
second we don't actually know how seastone bullets would work. Seastone, like the sea doesn't actually take away DF powers it just makes the user weak and we don't know how much seastone it would take to make a logia DF user too weak to change form. Then again it could work, it might just make the part of the body it touches unable to transform, who knows.
I do, however, agree with your point, logia user's aren't invincible by any means.
Get your manga facts straight. We do know that it actually prevents DF users from using their DFs. And we don't know if it actually weakens them physically. Cases in point:
(1) Smoker was not crawling around when he was imprisoned by Croc, he just couldn't get out 'coz he couldn't turn to smoke.
(2)Robin was walking around ok with seastone cuffs. She couldn't use her DF powers though.
(3)Ace couldn't escape either when he was in ID because of seastone cuffs.