A few weeks ago there was some discussion in this thread about using the space under the Jeep for storage or something. This is from an article in the January issue of Land Rover Owner:
I direct your attention to the second callout in the photo above:
GO WITH THE FLOW: the 70-litre fresh water tank is fitted internally, so there's no risk of freezing in harsh winters. There's an external locking filler in the rear quarter panel.
The water tank is fitted in the spare compartment under the floor (notice the spare now on the roof rack), so it's not truly under the vehicle, but it's an interesting implementation nonetheless.
The article says:
There's hot and cold running water. A heat exchanger in the engine bay feeds hot water to the tapon the rear door, while a connector on the front bumper receives a shower connector. Temperature is controlled via the Disco's heater controls while the engine is running, meaning Katrina can enjoy a hot shower any time, and literally anywhere.
Unfortunately there's no further information in the article about the water system, nor are there any further photos than the ones above.
What about something like this for the Wrangler? There isn't a compartment like that in the Wrangler, but there is some space underneath the Jeep. For those interested in mounting a water tank under a JKU, Dan Grec's excellent article is required reading:
http://theroadchoseme.com/diy-4x4-water-tank-pump-filtration-and-treatment. Everything you'll need to know about is in that article. Dan's Jeep is a 3.8L JKU and he says the dimensions under a 3.8L JKU should be checked because they may be different than his. I checked, and find these dimensions, the length available under a 3.6 model is less than Dan had under his 3.8 model:
The tank could possibly be taller than 6", but I'd recommend a skid plate under it if you have a taller tank. The fuel tank, which is on the other side of the driveshaft, has a skid plate with a height that might allow a 7" tall tank but you'd have to check.
There are a number of RV water tank suppliers and I did a quick search to see if there are stock tanks that might fit in the space under a 3.6 JKU. Here's one that should fit, although at 18.5" long it would leave almost 6" of length space unused. Perhaps with more searching a longer version of something like this could be found:
https://www.ntotank.com/5gallon-ronco-rv-water-tank-x4711116. It's only 5 gallons, if a 24" long version of this could be found it would be about 6 gallons, which is probably about all you could fit in the space under a 3.6 JKU.