Pour about 2 tablespoons of watermelon juice into a glass. Add mint leaves and muddle the mint (press down with a muddler or a spoon) until the leaves are fairly bruised. Add remaining watermelon juice, lime juice and rum. Top off with seltzer and serve with a spring of mint, if desired.
I found that a large watermelon yields about 10 1/2 cups of strained watermelon juice.