I'm pretty sure it's not the salt. I remember Oda saying that people with DF powers are also weakened by freshwater. Remember the time Kalifa was in the tub and Nami said that she must be weak?
That's not quite it.
According to an SBS, it's standing water that affects them. That means any water that's not in motion. Rain or waves don't affect them, but a water tank or a bath will. Crocodile is an exception because of his Devil Fruit.