I do not personally keep water water dragons however from reading the forum the minimum recomended viv size 5 ft long, 3 ft deep and 4ft high this makes sure that the lizard has enough space to turn around full and move. Your viv is a bit small as the lizard can not lay out fully extended this can cause problems as it will not be able to thermoregulate correctly. Nor can a thermal gradient be created. If you have a look through the "Habitat"section there are many impressive examples of suitable housing.
Hope this helps, Stephen