Check out this thread on my local Herp society forums - Rich (Brand X Rodents) built an AMAZING Nile Monitor enclosure, and if you message him (he is also on here) he may be able to give you some pointers on enclosure plans. His divides in the middle, so that he can expand it at any time.. and you can see in the photos how large it already is. That doberman is not small, and the aquarium they are filling is 75 gallons.

Keep in mind that your water can get even larger than a nile, and the enclosure Rich has built wont necessarily hold an adult nile or water for it's entire life.