Marine chemist, i wish to tender my short explaination sciemtifically as follows:
This slow mixing is driven by three factors
1 DENSITY: Density gradient of dissolved ions(tot. Salinity across).
The sea water may appear as if they don't mix, they do. What u see is a phenomenon called thermohaline circulation, where sea water with different densities delineate vertically..... The mixing process is slow tho.2 WIND REGIME:
The wind movement in alaska depend on coriolis coupled with ekman force where different water stratas circulate horizontaly and virtically at angle 45°. Causing minimal homogenization of water bodies. This slow mixing is also dependent on first factor above. Wind is one factor that controls water mixing from 1-down to near 500m. From the pic u can see it a calmer sea...no turbulence, no wave with shoals.
3 LIGHT
What u see clearly as boundary is light back scattering/reflection incident on the water body. Water bodies with more salt (tot ion) will appear mor turbid there by impeding light penetration. So ions with different salts allow reflection of light favoring wavelength tending to blue in uv-vis band. The clearer water simply means less salt and deeper light penetration, compared the the darker side. Its been proven that white light penetration is highest for blue.
My explanation doesn't mean there is no God. its meant to de-mystify and buttress further on earlier premise that they do not mix.

Where is the picture?

Anyway, the water bodies are mixing. It's just slow. And happens to be because of the difference in salinity,

Nothing "Divine" here.

If you want to see something truly divine, look at the human brain.