Beside silt skimmers, silt schooners (athasian emporium p 42-43) equipped with obsidian engines can travel through deep silt areas and function like “normal” boats, making them more suited to piracy.
These boats are however quite rare outside Balic, and need shipfloaters (psionicists) to maintain the ship afloat using the obsidian engine.
Enslavement in Balic could be a good start, with the PCs taken aboard a war silt schooner to do the difficult/dangerous tasks aboard.
The many islands of the estuary of the forked tongues can make good hideouts.
Giants wading into the silt make quite formidable foes, as do silt horrors, if you need to.
Ruktois (ToA p 135) can be used by smugglers which could interact with the PCs.
I had my players seek and destroy a gang of pirates operating a silt skimmer which posed a threat to the commercial route between Cromlin and fort Firstwatch (they were working for a merchant house).
I found a map of a balican war galleon somewhere a while ago, but I don’t remember where. Someone ?