Kent Smith, a.k.a Flickr user kentsmith9 claims to be the original photographer of this image.
I thought this was the most unusual thing I saw on the Alaskan cruise in the water. These two bodies of water were merging in the middle of the Alaskan gulf and there was a foam developing only at their junction.
I thought this was an example of a Halocline described on Wikipedia. A few people have commented that a Halocline is more of a horizontal phenomenon and this is more vertically oriented.
I am pretty confident that what you are seeing is a result of the melting glaciers being composed of fresh water and the ocean has a higher percentage of salt causing the two bodies of water to have different densities and therefore makes it more difficult to mix. I'm told they will eventually mix given enough time.
People have asked me if I just happened to look out over the edge of the ship deck and see this. Actually I had been on the deck for quite some time when I noticed what appeared to be a shadow cast by clouds over the ocean about 5 miles in front of the ship. As we approached the shadow I realized it was something different. I took many shots up to the point I shot this one, but never posted them until a year after this image went viral. I really posted them to convince people I did not Photoshop this image. See the other shots here.
He goes on to describe the recent viral spread of this image.
While Kent Smith is apparently not an expert in oceanography, he proffers a plausible explanation, and doesn't claim that the two kinds of sea-water will never mix.
The question asks "is it common or can it be counted a miracle?"
This is a false dilemma. Phenomena can be both uncommon and have natural, rather than supernatural, explanations.