In a small saucepan, over medium heat, add the balsamic vinegar. Bring to a boil, and then reduce the heat to a simmer. Simmer the balsamic vinegar until it has been reduced by at least half. Remove from heat and set aside.
On your serving dish, layer the watermelon, fresh mozzarella and basil in any order that you wish.
Drizzle the extra virgin olive oil and balsamic reduction on top.
Sprinkle with sea salt and black pepper to taste.
Serve immediately and enjoy!