Place the watermelon in a colander. Toss it with 1/2 teaspoon kosher salt. Let the watermelon sit and release its juices while you grill the corn.
Preheat a gas or charcoal grill on high heat.
Rub the corn with the olive oil. Grill, turning occasionally, until lightly charred, about 5-7 minutes. Let the corn cool slightly before slicing the kernels off the cob.
In a large bowl, stir together the watermelon, corn, cucumbers, scallions, cilantro, feta, flaky sea salt and black pepper.
Store leftovers in an airtight container in the refrigerator.
Notes
Store leftovers in an airtight container in the refrigerator up to 3 days.