Heavy water occurs naturally in all water in a proportion of about one part in twenty million. In order to get a certain amount of heavcy water you have to isolate the heavy molecules out of a large quantity of water. The Germans used fresh water because there would have been no point in having to desalinize before they could even begin isolating the heavy water from the regular water. Heavy water is heavy because one of the hydrogen atoms in the water molecule has a neutron in its nucleus along with the proton.

i believe that desalination can be done in several ways. the simplest of which is evaporation and condensation, which gives distilled water. it is ok to drink distilled water for about 2 weeks at the maximum. any more than that and your bones will weaken and you will get all loopy and you could die (seriously), because the distilled water is free from the minerals necessary to facilitate healthy bodily chemistry, your body WILL find its minerals from somewhere else.

you could also desalinize water with chemistries, all without leaving the liquid phase. that would remove the sodium chloride and ideally leave everything else unreduced.

What do you mean "Use oceans?" Like, fishing? Travel? Defense?

We use the oceans for many reasons. The oceans are a good source of fish, thus bringing in revenue. They also are amazing vacation destinations, and if you are shipping large quantities of something overseas you use the ocean for transport. (I.E. Oil going from the Persian Gulf to America, Europe. Chinese-made prducts being shipped to the USA.)

The real question is, why don't we use the oceans more? They offer plenty of energy, water (If we can desalinize it), and space to build homes. The US government has an oceanic exploration division, similar to NASA, called NOAH. I is largley underfunded, much to the anger of many people. The oceans should be the first place we explore, as it is likely more economical than space.

Many methods for seawater desalinization have been used for a number of years starting in ancient times with good old fashion distillation (boil the water and catch the condensate). The leading method now is membrane based...reverse osmosis. Expensive to build, expensive to operate and maintain.

New studies are underway to improve membrane solutions include forward osmosis which actually uses an ionic salt process and then removing the special salts from the solute..or good water side..of the membrane process.

Carbon nano-tubes built into membranes and electrically charged to repel salt ions before reaching the membrane, and biomimetic membranes utlizing aquaporins in a similar charged fashion hold some promise to improving the efficiency of RO systems but are still in the theory and development stage.

One new process that is actually in commercial development is Capacitive De-ionization which uses a flow-through capacitor designed to eliminate dissolved solids from water using a small electrical charge. Read more about his here: http://ggammonewp.wordpress.com/

The desalinated water is more expensive but in many countries is absolutely necessary.