For the marinade, in large enough bowl to transfer the watermelon balls to eventually, whisk together the Rum, lime juice, brown sugar and salt. Set aside.
Using a melon baller, scoop the fruit from your half of watermelon, transferring it into a colander as you do so. You don't want this salad to end up watery. If you don't want to use a melon baller, just cut your watermelon into small chunks instead.
Once the watermelon half is empty, tip over the sink to drain it of excess liquid.
Now that you've got your watermelon half completely empty, transfer your watermelon balls to the marinade and top with lime zest and mint. Stir gently to combine then transfer back into hollowed out watermelon half to serve immediately. Enjoy!
****NOTES****If you're not planning on serving immediately, do not mix up until right before you do serve. Watermelon breaks down easily and you don't want this to get mushy. Also if you end up having leftovers, make sure you drain it of all the excess liquid before storing for that very same reason. Since watermelon breaks down in liquid it is best to try and eat this the following day if you do have leftovers. BUT, why would you have any leftovers? This stuff is AMAZING!!