In a small saucepan, combine 1 cup water and the honey. Bring to a boil and shut off the heat. Transfer to a bowl to cool. Refrigerate.
In a medium bowl, combine half of the cucumber slices with the grapefruit segments and some simple syrup. Muddle them by pressing down on them with a whisk to extract the juices and lightly crush the cucumbers. Add the vodka, lime juice, some more simple syrup and 1 cup lightly crushed ice. Pour into 4 rocks glasses and grind a little salt over the top. Alternatively, serve in a pitcher and sprinkle with the salt as you pour the individual drinks.