Drink if you're thirsty. That's what I do. This 'must drink x amount of water a day' is BS.
Because yes, running to the toilet isn't doing you any good. When sick they say to drink water so your urine is straw colored. Who wants to keep running to the toilet when you're sick. It's terrible to have the flu and feel dehydrated but once you can keep fluids down it will correct itself. Of course the older you get the more risk you are for dehydration.