Chop watermelon into bite size chunks and place in a large bowl. Sprinkle sugar over water melon and toss to coat. Transfer the watermelon to a colander and put the colander back in the bowl. Place in the refrigerator for 30 minutes. Discard watermelon liquid.
Chop cucumber in ½-inch dice, toss with 1 tsp kosher salt and toss. Put cucumbers in a collander and place it ove r the bowl and put in the refrigerator for 30 minutes. Discard cucumber liquid.
In a small bowl, whisk oil, vinegar, and ½ tsp. kosher salt. Add watermelon, cucumber, feta, mint, and onion and toss to coat in dressing.
Top with more mint and sea salt (if using).
