Cut the watermelon into wedges and place it on two baking sheets in a single layer.
Place the water, sugar, lime juice, rum, mint and the zest of 1 lime in a saucepan. Heat over medium heat, whisking so the sugar dissolves. Cook until just simmering then remove it from the heat and let it cool to room temperature. Pour the mixture evenly over the watermelon in the baking sheet. Move the watermelon wedges around so they are submerged in the syrup, then flip them.
Refrigerate for 30 to 60 minutes. I like to flip the watermelon once while it’s in the fridge.
Remove the watermelon from the syrup and place it on a platter. Sprinkle on the remaining lime zest. Sprinkle on a bit of flaked salt and serve!
As a note, you can do this on an as-needed basis. Just use a few slices of watermelon and pour some of the syrup over top, storing the rest in the fridge until ready to use.