Its been years since I had a water dragon, but I think the most sensible place to start is a good herp vet. If you have no history of the animal you need to get professional advice as to what is wrong / how to get it back on track.

Is it wild caught or captive bred? Whats the length (snout to vent)? Temps / humidity / enclosure info? Water dragons need a good amount of space, so the tub cannot be a permanent solution.