I suspect you are having a pathing problem where your hungry/thirsty dwarves are unable to get to that part of your fort that has food and booze.

Water is only drunk by injured dwarves; they normally prefer to drink booze. Diagnosing and other medical labors are only needed on injured dwarves. Dwarves that are hungry or thirsty are not injured (at least to start with), so medical labors don't apply unless they are very far gone.

If your dwarves are injured or otherwise unable to move, and if your other dwarves have access to a bucket and a water source, can path to the thirsty dwarf, and have the "feed patient/prisoners labor turned on, they should bring water to them. If you have designated a hospital zone, your other dwarves have the recover wounded labor turned on, and they can path to your injured dwarf and to the hospital, they should haul the injured dwarf to the hospital.

Designating a water supply zone at a pond or river or if you have built a well (which is automatically considered a water supply location), your dwarves should use that if they have a bucket and need to water an animal or an injured dwarf.

[More specifics about taking care of injured dwarves.][1]


  [1]: http://gaming.stackexchange.com/questions/5169/how-to-tend-to-a-wounded-dwarf?rq=1