Using the Treasure Thing isn't as straight forward as the other two tools the Sea Treasure Hunter gives the player.
Depending on the player's distance from a Cursed Sea Treasure Cache, the orb atop the Treasure Thing will change color, varying from a pure black up to a bright red, indicating a cache is near. The radius at which the Treasure Thing detects a cache is ~20 nodes, so it can be relatively hard to located them. However, once the player is ~10 nodes away from a cache, the large pillar of light indicating its location should be visible.
The following is a somewhat inaccurate visual representation of the colours the orb can be and the distance to a cache.
Once the player is within ~4 nodes of a pillar of light, however, it will vanish, making Cursed Sea Treasure Caches a very difficult item to obtain. If the player manages to position their Treasure Grabber directly above one, they may use it to acquire the cache and loot its contents.