In a medium saucepan over medium-high heat, bring water, sugar, cranberries, and grapes to boil.
Reduce heat to medium-low and simmer until most of the cranberries have popped and the grapes are soft and turning translucent, about 15 minutes. Stir occasionally. I like to mash the grapes against the pot with the back of a wooden spoon so they get broken up. Leaving them whole works as well.
Remove from heat; add salt and stir. Let cool to room temperature. The compote will thicken as it cools down.
Serve at room temperature or chilled.
Notes
This recipe can be prepared up to 1 week ahead and stored in the refrigerator.