Place half the watermelon in a pitcher and mash well. Add the remaining watermelon and the rosemary, then fill with 1 1/2 quarts of cold water.
Refrigerate up to 3 days and enjoy!
Notes
If you wanted an even sweeter drink–like a lemonade–you could add some simple syrup. Hmmm…that’s making me think that watermelon lemonade would be quite tasty, too!