Recovery of coal from RMS Titanic how was it preserved?

Reading your commentary, I see that your question is predicated on the idea that coal exposed to seawater would dissolve. This seems reasonable, given that there are microbes that have destroyed a great deal of the wreck, to the point where it is generally believed that in another century or so, nothing will remain of the wreck, that the huge sections of steel hull will gradually dissolve. However, it appears that the microbial life-forms that have attacked the steel, wood, and other substances making up the wreck have been less devastating to the coal that is scattered on the ocean floor, so that the coal remains intact.

When the Titanic was sinking, water flooding into the forward portion of the ship drove out the air that had been there, thus greatly decreasing the buoyancy of this part of the ship. The tremendous weight of the front of the ship eventually caused the ship to split into two huge sections. Once it broke free, the front portion apparently plunge downward at a rate that may have hit 40 miles per hour. It hit with such force that it prove itself some forty feet into sea bottom mud, and a massive undersea wave caused by a slip streaming effect rushed over this portion of the wreck, severely damaging the bridge and other portions of the wreck. The rear portion of the ship sank, but because there were air pockets in this section, it did not go down as fast as the front section, and drifted in deep currents so that it came to rest on the sea floor about a half mile from the front section.

In the area near the break, coal bunkers were torn open and a large amount of coal poured out. This coal became the third principle debris point, somewhere between the two portions of the ship. The fact that the coal is in a relatively discrete area suggests that one or more coal bunkers tore open in a single, wrenching action, dumping tons of coal into the water apart from the stern section as it sank.

Coal will remain intact in sea water for a very long time, so that there is still a large amount of coal on the sea floor. The most common way that coal is destroyed is through combustion (burning) or a combustion-like chemical reaction. This is virtually impossible because sea water at a depth of 12,000 feet contains virtually no oxygen.

There is an additional reason for retrieving the coal rather than other artifacts from the Titanic: The bodies of 1,300 people were never recovered. While it is impossible to determine an exact number of the bodies that sank with or near the wreckage, it is believed that the two massive sections of the wreck are the final resting place of many people who were unable to escape the doomed ship that night. While exploration of this wreckage may yield a limited amount of scientific data, both common decency and international compact have put restrictions on the techniques that are now acceptable for the treatment of the wreck of the ship proper. By contrast, there is less possibility that the coal field contains or contained human remains, so that fewer restrictions apply.

Much of the coal retrieved from the wreckage has been auctioned off to collectors. In this regard, the coal was treated as distinct from other artifacts. The company that conducted the salvaging of the artifacts and which controls the salvage rights has stated that no other artifacts will be sold; everything will be retained for museum display.

The coal was a HUGE debris field. Essentially, coal is carbon, so immersion in water won't affect it. As the Titanic split and sank, the the boilers broke loose and broke through the sides of the ship allowing the coal stored in the bunkers to fall to the sea bed.