I'll agree that from your description of him being lethargic and not eating that it sounds like something is wrong. How you got from there to 80% sure he has parasites, I don't know.

I very strongly recommend against giving a dewormer when you don't know your animals has worms, or what kind of worms they might be. I don't know about panacur in particular, but most dewormers are POISONS. The dosage is determined by figuring out how much is enough to kill the worms, but not enough to harm the host. Of course, we don't really know if it is harming the host or not, we just know that we don't notice any harm. Although it is common to routinely give dewormers to many animals, whether they need it or not (heartworm meds for dogs, broad spectrum dewormers for horses), this is something I view as a necessary evil at best, and in some instances, I wonder how necessary it is.

With our reptiles, we are lucky, because they are pretty isolated. A dog is at risk of getting heartworm every time it goes outside during mosquito season, or even if a mosquito gets in the house. That same kind of risk just isn't there with out reptiles.

The worst thing is, if your treat your water dragon with a dewormer when he is sick with something totally unrelated to parasites, not only are you not doing him any good, but you are possibly going to make the situation worse by causing stress when you administer the medication, or due to the stress the dewormer causes on his system.

You really should take him to a good herp vet, and find out for sure what is wrong with him, so you can give him proper treatment that will help him get better.