Whisk marinade ingredients together in a bowl then transfer to a plastic Ziplock bag set inside another large bowl.
Pound chicken to an even thickness then score diagonally across the top and place inside bag with marinade. Marinate in the refrigerator for 30 minutes or up to 2 hours.
Grill chicken breasts over medium-high heat for 3-5 minutes a side, or until no longer pink in the center. Let rest for 5 minutes before serving.
For the Watermelon-Cucumber Salsa:
Combine ingredients in a bowl then spoon over grilled chicken.
Notes
This marinade is also great for steak, pork, or salmon.