Raptors do not need to drink most of the time, because their diet contains lots of moisture. This means that giving medication in drinking water is useless.
This does not mean that you should not provide water- most raptors enjoy a shower or bath when they are feeling comfortable and secure in their environment. Unfortunately, many will not drink in a hospital situation, where they are in an unfamiliar environment. This presents a problem for rehydrating or giving some medications.
As with all drugs, make sure the raptor is hydrated before starting. This may mean force feeding the bird fluids through a stomach tube or crop needle. A stomach tube is preferred. Some medications can be added to the fluid being tubed, for example, Ronivet for trichamanosis. Raptors are very different to rehydrate than honeyeaters or water birds, who drink readily. They will also aspirate (inhale) fluids much more easily than something like a dove, so care must be taken. For those carers who have not used this technique, you should either get someone with experience to show you, or use the 'drop at a time' method. The problem with giving a badly dehydrated bird a drop at a time is this increases the amount of handling time a lot, and will result in more stress on the bird.
Powdered drugs that have a low toxicity (as with many that are given in drinking water) can be sprinkled into moistened food. For example, Hapavet is a worming drug. Instead of diluting into drinking water, a dose is placed on a mouse neck, under the cut skin. The powder is moistened by the meat, and will be eaten by the raptor along with the food. This can also be done with liquid medication such as antibiotics. Baytril can be injected into the dead food animal, or into a bite of meat. Unfortunately, this common drug (and many others) is not usually palatable. In the case of a patient that will avoid eating it, the dose has to be put into a bite of food, and this is hand or force fed to the bird before the main meal. Tablets or capsules can be placed in food, but care has to be taken to make sure they aren't being discarded. In this case it is much easier to place the tablet in the back of the throat and immediately feed the bird.