Briny Chalice

A simple ceramic chalice. By first filling the chalice with sand and then pouring water into the cup the sand will dissolve. When all of the sand has dissolved the chalice can be drunk from, with the water being salty if there is undead nearby. The intensity of the saltiness determines the strength and distance of the undead.
Sailors of the Saltmarsh know to be wary of creatures that defy the grave both above and below the wave.