I thought the SUV you rented is funny. I think it has a snorkle on it for engine air, but then they don't want you taking it off road? I think they might be marketing to the tourists with that thing. :-)
Actually, yes and no. It is possible to drive through water deep enough to require a snorkle without going off road, but only during the wet season. I didn't get around to posting about the shortcut we took on the way back from King's Canyon yet but it was a gravel road and we drove through at least two river's (sans any water) and they would have both been deep enough to need a snorkle had there been water.
The short cut saved us just about 150km, but was over heavy washboard on gravel. We saw wild camels though and like I say it probably saved us a couple of hundred dollars in fuel and excess kms.
The river in Alice Springs and I presume every river for 500km in every direction from Alice Springs is dry 99.9% of the time.
It rained our last night there from about 10pm untill at least noon when we left and puddles were forming in the river and one of the main runoff channels was actually turning in to a large creek.