Freeze Sea-Water to Convert it into Freshwater?

As sea water freezes, dissolved salt is excluded, and the resulting ice has much less salt in it compared to sea water. Repeated recycling between solid and liquid can eliminate virtually all the salt in sea water.
Salt content of different waters
No water is totally free of salt. Sea water has a typical salinity of 3.5%.

Water salinity based on dissolved salts
Fresh water	Brackish water	Saline water	Brine
< 0.05%	0.05% – 3%	3% – 5%	> 5%

Crystal Structures
Ice and salt crystals have very different structures in solid form:

Crystalline Structure of Ice
Source - ps.uci.edu

Crystalline structure of  Salt
Source - webelements.com

Ice has a hexagonal (six-sided) structure while salt has a cubic (four-sided) structure. It is primarily due to this structural differences that salt and water separate during the freezing process.

The Freezing Process

As pockets of sea water start freezing, the crystalline structure of frozen water begins to appear which pushes salt molecules away to produce water pockets rich in salt (aka brine solution). As the salt-free water freezes, the brine solution resists freezing because its high salt content has lowered its freezing temperature.

As ice freezing continues, this brine solution, essentially, leeches out from the water crystals leaving behind water whose salt content is much lower than that of sea water.
Freezing speed, if fast enough, can trap some salt molecules inside ice crystals but these are gradually released as ice crystals reach for their steady state.

Freshwater - A Singular Resource

Singular, according to the American Heritage Dictionary, means "Being only one; individual, Deviating strongly from the norm." Yes, water, and in particular, freshwater is unlike any other resource.


Water is MORE than just another Commodity

Commodities
Source - tradecommodity.com.au

Water is a commodity in the sense that it is a material that can be bought and sold. 
Being basic to all life is what makes water a commodity with a difference.
Not only was water the medium for incubating life, water is also continuously required for sustaining and maintaining life.
We cannot, therefore, just think of buying and selling water - we must also factor in how humans, all other living species and nature's ecosystems are impacted by us moving water around.


Water is Distributed WORLDWIDE

Freshwater Distribution
Source - rusrev.org

Water is the most widely distributed substance on Earth. It is available everywhere. While the specific amount of water differs in different locations, there is no place on Earth with, literally, no water.
All other natural resources are also widely distributed, but each and everyone is not available everywhere even in the smallest of amounts. Some amount of freshwater is available everywhere on Earth.


Water has NO Substitute
There is no other substance, natural or synthetic, that can replace water. It is hoped that the world will migrate away from fossil fuels to renewable energy. No such transition is possible with water.


Water exists simultaneously in multiple forms: Vapor, liquid and solid

3 forms of water
Source - fcwa.org

No other resource exists in more than one form in the same location. Huge amounts of water are also bound into soil and other materials in every location.
Every other resource exists in a single form, usually as a liquid, in the same location.


We cannot MAKE new water

Periodic Table of Elements
Source - elementsdatabase.com

We humans cannot make any element of the periodic table or the basic compounds that combinations of these elements form in nature.

Water formation Equation
Source - scienceforums.net

Making water is a particularly tough proposition because of the amount of energy released. We simply have no way of containing this energy release.


We cannot DESTROY water
When we burn oil, no more oil remains, as the oil disintegrates into different chemical compounds.
Water, on the other hand, does not ever disintegrate into its components. Water gets polluted, changes color, acquires an odor or simply evaporates into the atmosphere or disappears into the ground.


Water is in Constant MOTION

Water Cycle
Source - kidzone.ws

Unlike every other natural resource, water is in constant motion. It is constantly moving between its three forms - from liquid to vapor, from vapor to liquid, from solid to liquid and every other possible transformation. Water is always moving in rivers and churning in lakes and oceans.
NO other natural resource exhibits this perpetual motion behavior. 


Water's Singularity
Water's perpetual motion, it's simultaneous existence in multiple forms, it's availability everywhere and its role as the basis for all life make it singularly unique.


We seem to want to treat water in ways similar to how we think and act with other resources. This may be our greatest error in dealing with water, and especially, finding ways to alleviate water scarcity everywhere on Earth.


We should, instead be looking at harnessing the unique characteristics of water to end water scarcity everywhere on Earth.