PBA -“The question is what if I need to water to avoid being uncomfortable.”
This is your mistake. Your situation of hiking, esp. in hot weather -there is no way to know if your hunger/thirst pangs are due to your obesity (if you’re fat) or due to bodily needs. So since there is no way to know -you’d be Mechuyav to eat and drink.
I don’t know even I ran tests -if I’d be able to tell. So now you’re in a Sofek Sakana whether you realize it or not.
Whether you’re allowed to go hiking without water to wash in the first place is something you have to ask your LOR!