Okay, this is mostly to strengthen the Water Devil Fruit theory.
Isn't the main thing about the devil fruits is that SEAwater weakens the user, this nullifying their powers and not letting them swim. SEAwater. That means salt water. There's freshwater and purified water out too. Do those weaken them too? Cause if so, then yeah, no water DF as that would be OP as fuck but if they're using freshwater or purified water, then it would be fine.