Relatively easy achievement despite the description of it.
You'd THINK the whole car/truck needs to be submerged, but in reality, just the cab does.
I got the achievement on the challenge map "Night Safari".
You start the map in the big truck (yeah, can't remember the name, but it's the 8x8 truck), all you need to do is turn round, and following the road for a few seconds, there should be a swamp like lake on the left, just dunk yourself in there until the cab is submerged and reverse out. If it doesn't unlock the first time, just rinse and repeat until it does.