So, I contribute to the One Piece wikia and was thinking of editing Sanjuan Wolf's article and stating that he is a confirmed non-Devil Fruit user, based on Chapter 595, but upon closer inspection of the chapter, I've come to realise that it's not so clear after all.
Here's the page in question:
http://read.mangashare.com/One-Piece/chapter-595/page006.html
Sanjuan Wolf: "Ahhhh! / I can barely keep working my legs... / No power'll fill my body... / Oww! I was bitten by a fish!
Now, after reinspecting the page, I have two possible opinions, as opposed to one definite one.
1. Sanjuan Wolf is not a devil fruit user, because he is treading water in the ocean and not drowning. The fact that he needs to constantly tread water means the water is too deep to stand in, and since he's keeping his head well above water, he is not in fact drowning, so he could not have eaten a devil fruit, meaning he is just a super huge giant. Since he also says that there is no power in his body, this could just mean he's tired because of the constant paddling.
2. He may be a Devil Fruit user. It's the line about there being no power in his body that makes me think this, since Devil Fruit users feel powerless in water. This could mean he ate a supersize devil fruit, and his body has been permanently altered like Luffy's, so it's his enormous size that allows him to not drown, since maybe the water is only deep enough that his feet shouldn't touch the bottom, so he just needs to keep his head above.
We've seen that if a devil fruit user's body isn't completely submerged, they aren't completely powerless, and this has been evidenced by Kalifa and Hancock in their baths, and Luffy when he, Smoker, Zoro, Usopp and Nami were in the cage under Rain Dinners Casino and the room was filling with water. So basically what I'm saying is, Sanjuan is feeling drained from being in water, but isn't completely submerged so he still has enough energy to tread water.
Also, perhaps his size has nothing to do with his ability, perhaps he could have a totally unrelated ability and his size could be natural.
So what do you guys think? I think each side of the argument is equally compelling at the moment. On the one hand, devil fruit users can't swim at all, not even paddle, so the fact that he is able to tread water and keep his head above water should mean he is not a devil fruit user. On the other hand, the fact that he is complaining about being powerless is reminiscent of devil fruit users. Hmm...