No but seriously, someone of Wolfs size shouldn't drown if he transforms a couple of meters away from the shore. I doubt that he would even be submerged in water enough to be weakened.
Haha, that's a good point. In shallow enough water, which is pretty much anywhere for someone his size, he should be able to stand up and walk. Still, I have my doubts.
What if he's […] a ship that somehow absorbed the power of […] a (giant?) devil fruit?
Hmm. While this is certainly plausible, it rides on whether or not other variants of that type of fruit exist, which I am leaning towards no. All the Zoan's that had variants were "models"; ie: Inu Inu Fruit: Model Jackel, etc. The Zoan's that do not have variants lack models; ie: Uma Uma Fruit. Chopper's fruit (Human Zoan) was one of the latter, lacking a model. Had Oda planned to make other variants, he would (should) have planned ahead by making it something like, "Homo Homo Fruit: Model Human" and "Homo Homo Fruit: Model Giant" or some such. (Referring to the genus 'homo'). Other than that, Devil Fruit items are a recent invention, so him being in prison would have to be a fairly recent thing…