There are two ways used today. You can distil the water away from the salt and condense the pure water from steam using a cooling coil, or you can use a Reverse Osmosis membrane and push the water away from the salt.

There are two ways. The first way (which is used for large scale desalination) is reverse osmosis. To understand this process, you first need to understand what osmosis is. Let's say that you have two units of water that are side by side. One container is filled with fresh water, the other with salt water. The only thing separating these two quantities for water is a thin membrane that has holes in that are big enough to let individual water molecules pass through it but small enough to keep the sodium and chlorine atoms of the salt from passing through. If there is nothing else working on these units of water, then the fresh water will tend to mover through the membrane and into the salt water. This is osmosis. Now then, if you rase the water pressure on the salt water side of the membrane you can force the water to reverse its direction of travel. Thus, you can force the water to go from the salt water side to the fresh water side. since the wholes in the membrane are too small to let the salt through, all you get coming out is the fresh, desalinated water. This process is reverse osmosis. The second method of desalination is through the distillation process. In this process, you take salt water and make the water in it evaporate. You then cause this water vapor to recondense someplace else where the now pure water won't mix with the salt water. One way of accomplishing this is with an old time still. With a still, you have a tank filled with salt water. underneath this tank you have a heat source which causes the water to evaporate. The tank is completely sealed so that the water vapor thus produced must pass through a long coiled tub in order to escape. As it passes through this coiled tube, the vapor cools and much of the water turns back into liquid. This now pure water runs out of the coil at the bottom end where it can be collected in a container. That is a description of the type of still that might be used in a laboratory. However, I have also seen plans for solar powered stills that one might use in survival type situations. As this is the section on ships and boats, I will describe one that can be used in life rafts. This type is made primarily of some kind of flexible plastic (vinyl for example). It inflates so that you have a dome of clear plastic sitting on top of the water. Sunlight comes in through the dome and cause the water to evaporate. This water vapor then condenses back out onto the inside surface of the dome and runs down the sides to a collection trough from which the fresh water can then be drained for use as drinking water. To find some actual examples of this type of still do a Google search using "Solar still nautical".

go to http://pbskids.org/zoom/activities/sci/waterfilter.html

Here is a pretty self-explanatory diagram that i think might help. It helped me a great deal.