The Shifting Sea is comprised of silt so fine that it is incapable of supporting any but the most negligible load upon its surface. The sea is 8-12 feet deep in most places, although, in isolated spots it can be much deeper. Airships can fly over the sea, but are generally useless for cargo and extremely vulnerable to the frequent sandstorms. Large land animals (e.g., zitidar) can navigate the more shallow deposits of silt, but have difficulties with respiration after just a few hours of exposure. Sand ships are the safest and most reliable means of conveyance across the Shifting Sea. All known salt mines in Stark's Reach are located on islands (areas of high elevation) within the Shifting Sea.
Note: on the current version of the Stark's Reach map (left sidebar) the Shifting Sea is mislabled as the Lesser Sea of Sand. Also props to TSR/WotC's Dark Sun setting which provided the inspiration for the sea itself.