By the 2e books, silt can not support a creature's weight - you might be able to wade some distance out, but you can't swim in it. If it's too deep, you'll start to suffocate. At a depth of about 15 feet, it starts being compacted enough to support some weight, and in places there are paths about 12 feet below the surface where the weight of giants have compressed the silt enough that they can walk on it (but that's little use to most PCs unless they can find giants to carry them).
The most common ways for non-fliers to cross the Sea of Silt is by means of silt skimmers - relatively small craft with ginormous wheels which they use to roll along the same paths giants wade on. These are usually labor-powered, because if there's enough wind over the Sea to power a silt skimmer, there's enough to stir up the silt in the air and reduce visibility to a few feet.