Diarreah worsens the dehydration because it is further loss of fluids. Try adding water to anything that will inspire him to eat that is not salty i.e. NO FISH! Tuna and turkey is salty and further dehydrates. try chicken livers from the meat section (boil on stove and give with broth) or open a can of chicken broth- low sodium if you have it! Anything to tempt him to eat, and that you can add some water to and it will still be tasty. Roast beef is a winner- if its rare- at least the moisture is a bit of water...
And yes dehydration can kill quite easily. vet is needed asap even if you need to use emergancy vet, you should. Go to thread re: lucky/cheach. He could have died monday night had not dusty's people brought him to a vet. He was dehydrated and high fever- fever was unknown because nose and ears still cool.. We can't make diagnosis at home!