No it's radically different. If you haven't eaten enough food your body will burn fat for energy. But your body has no alternative hydration source. You can go a month easy without food, you cannot go more than a few days without water and still live. And you cannot go a few hours of being lightly dehydrated without your body's performance falling below optimal

Just because you don't die doesn't mean your physical and mental performance aren't reduced. You absolutely cannot go days without being dehydrated

The first signs of dehydration are not headaches and muscle cramps lol that's when you're already well into it. Your physical and mental performance will have already been reduced by the time you feel thirsty. If you're correctly hydrating you won't experience any symptoms whatsoever