I wouldn't, but you probably could. See Cheechako's post. Maybe better to keep both water tanks, and just add a fuel tank or two, if you want more range. A water maker is nice, but even nicer to have enough tankage that if the watermaker
stops cooperating, you still got plenty. And should you ever want to sell the boat, a potential buyer might call that a black mark against the boat, having half the water capacity of a stock model.
Some sailboats don't even HAVE a fuel tank, or an engine
. So obviously, fuel capacity is an option. Water isn't optional, IMHO. It is too important to be at the mercy of an expensive gizmo that ought to work as advertised, but might not. My opinoin only, and no disrespect intended if you feel opposite.