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.
Injury
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.
Pathing problem
You can test whether the dwarves can path to food or drink. Try to assign one of your other dwarves to chop a tree down or some other job near your stuck dwarves, like setting a pasture, assigning an animal to it, and have the dwarf lead it there. If that dwarf can reach the tree or pasture, there is a clear path to the inside of your fort. If the dwarf just stands around, the game may be unable to find a path there.
I had a bugged game like this where I could see no problem pathing between inside and outside my fort but DF was unable to find a path in that game. It may have been related to the reclaim of an abandoned fort.
