Here is a simple definition of a hypertonic solution:
"A solution with a higher salt concentration than in normal cells of the body and the blood." As opposed to an isotonic (same, similar) solution or a hypotonic (lower) solution. The last item in the heading is just to generate some more interest in various potable solutions (make mine Gordon's).
In the sea, most ingredients are water molecules (below left) which consist of one oxygen (O=) ion and two hydrogen ions (H+).
These are configured so that there is an angle of
about 104.5 degrees between the centers of the oxygen and the two hydrogens.
Consequently, there will be a small positive charge on the side of the hydrogens and negative on the side of the oxygen.
This makes water a bi-polar molecule which accounts for many of its unique properties, including that of hydration (below).
It also allows one to use a Mickey Mouse face (oxygen) with two ears (hydrogens) to illustrate the water molecule.
In hydration, cations (positive, metallic) like Na+ and K+ will be surrounded by water molecules presenting their negative poles - which make them appear like some types of Japanese dolls with multiple heads on a single body. (especially the K+ in the figure)
I did not show the copy-righted figure showing both Na+ and Cl- ions, the most common "salt" in seawater. . Suffice it to say that the water-molecules surround the negative or anions presenting their positive side. This does away with the Mickey Mouse analogy because the "ears" now look more like wheels standing on the body of the anion.

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