[Jack Simth]

Originally posted by Kamog:
That's a handy modification they did to that truck. Maybe they can drive along, and when they come to a river, just keep driving into the water, chug across, and then drive up the other side. That's also useful in case of floods.

You can actually do that with most vehicles.
The modifications needed:
1) Seal certain components of the engine, including all of the electrical ones. Many engines come this way anymore; it prevents your engine from going kaput if you open the hood in heavy rain to give someone a jump or drive through a puddle too fast, as well as allowing for some nifty demonstrations on infomercials.
2) Raise the engine's air intake and exhaust above where the waterline will be, and seal that route. The routes are normally sealed already, but they will need to be raised in order to avoid drowning the engine. Raising the exhaust is only nesessary if you need to be able to start the engine while in the water.
3) Add a propeller, hooked up to the transmission in some fashion. This one is rather self-explainitory. It is often easier, although less convienient to run, to add an outboard motor.
4) Add boyancy. This is not needed for some cars, such as the old VW bug (while an old VW bug will sink eventually, it takes a while).

If those four (rather unspecific) steps are completed successfully on a well-running automobile, you will have an amphibious vehicle.

Note, however, that more work will be needed if you don't want to get your feet wet while on the water.

However, it is usually less time-consuming to find one of the ready-made amphibious vehicles, although they are usually horrifically expensive. On the plus side, they allow you to go boating without getting your feet wet.