Laughing Zells

To the South, in the Sea of Ghosts, there dwell a tribe of Zells who never leave the frightening transluscent sea, where the bottom of the ocean — and its terrible occupants — are visible from the surface.
This tribe, called The Laughing Zells, famously trade secrets for treasures. When approaching for a trade, a ship must fly a red and white flag. The Zells will draw close, and take one of the crew aboard to speak to them.
The secret told must hurt to tell. It is not clear what metric the Zells judge the value of the secret on.
The treasures they provide vary in value as well, but all are unique. A knife that can only cut wood, but which slices through it like butter. A piece of string that can never grow knotted. A lump of steel that can be molded like clay when submerged in water.
These treasures are mostly sought after by the wealthy as curiosities, as they only rarely have practical value. Venturing into the Sea of Ghosts is dangerous at the best of times, though, so few such treasures are in circulation at any time.
If a secret doesn't suffice, the Zells laugh and send the whisperer away. They're very cheerful people, supposedly. Their ships are unusually built as well. The Sea of Ghosts has "light" water, it's almost impossible to swim through, and rumor has it you can actually breathe the water there, like air. 
Their ships have enormous water displacement so as to stay afloat even in those strange conditions, and they almost seem to glide over the water, barely touching it.