I used to use the square food-grade Willow 25L water containers that had a tap. You can take the tap off and seal it with the bung supplied in the lid (you take the bung out of the lid before filling it, then keep the taps handy). This lets you move them around to distribute the weight evenly, you can cart 'em over to where the water is if needed - very easy and inexpensive.
Nowadays of course the 'van has a pair of 59L tanks underneath, but I still take a couple of cordial bottles (made of the soft plastic, not the PET stuff) in the Engel in case we need some cold water.
There is plenty of room under the tub. The wall spaces alone (between the outer and inner skins of the tub) would hold 40+ litres on each side with ease, and I remember someone either selling or making flexible bags that used these spots.
If you were just taking the Navara and using tents/swags (whether a roof-tent or a peg-in-ground type) and not towing anything, you might consider filling the voids under the tub with water tanks.
I'd suggest four things if you do it...
First, make sure they're mounted in such a way that the tap will allow every drop of water to be emptied from them.
Second, when you've finished with the tanks, pour a strong bicarb of soda solution into the tank, drive it around and empty it out. This keeps the tanks clean and the water untainted.
Third, don't rely on those tanks for fresh drink-from-the-tap water when you're in the desert regions. At christmas, our tanks - despite having a shield - got so hot we could shower directly from the tanks without using a water heater.
Fourth, be aware of how much weight they add to your vehicle. 1 litre of water weighs 1kg, and the tanks will weigh a little too. Don't overload the vehicle!