Luffy as a water balloon We discussed a lot about drowning devil fruit users in a bath - see this question: Can a Devil Fruit user drown in a bathtub?. 

So now I remembered the fight between Luffy and Crocodile. To have a chance, Luffy drank a lot of water and was like a water ballon. But why didn't this affect his strength?

Does he have to be surrounded by water?
I think he isn't affected, because technically at that time, he isn't submerged in water, but the water is rather submerged within him. According to the rule, this should be fine. If it wasn't, Devil Fruit Users would always die of thirst or even die from the water within their own body. This type of water could also be considered "moving" water, as it moves constantly with every move you make. The same goes for when Luffy defended Whitebeard against Crocodile at Marineford. At that time he covered himself with water and was still able to move freely, since it would be considered moving water. So unless they are submerged into it, Devil Fruit users can touch water without weakening.

From the Devil Fruit wiki page.


  With this new power comes a weakness: the sea rejects the new Devil Fruit user and they become helpless while submerged in water (or "become a hammer," in Japanese parlance).[2] Oda stated in SBS that Devil Fruit users are susceptible to all types of water, not just seawater. This includes the white sea surrounding Skypiea. He elaborated on this, saying that "moving" water, like rain or waves, does not weaken Devil Fruit users, while standing water does.