This might be stupid idea but I will give it a shot.
Well as we know haki negates devil fruit powers.(shown how rayleigh hit that light guy). SO I have a idea,since luffy can use both haki and a devil fruit....cant he negate his own devil fruit power for a short period of time in order to swim.That way his only weakness- water, wouldnt be his weakness anymore.
Tell me what you think xd
