About an hour before you're ready to grill the watermelon, combine the sliced red onion, white vinegar, lime juice, honey, and salt in a small glass bowl. Cover and refrigerate for an hour.
When you're ready to cook, preheat your grill to High (about 500 degrees F). Slice your watermelon into 1.5 inch rounds using a large, sharp knife.
Brush each side of the watermelon slice with the liquid from the pickled onions. Place the watermelon rounds on the grill and cook for 3-4 minutes per side, or until there are some light char marks on each side of the watermelon. You might need to work in a few batches at a time to get all of the watermelon grilled if your watermelon rounds are large and your grill grate is small.
Place each grilled watermelon round on a large cutting board, drizzle with some of the honey, then top with a few slices of the pickled red onion, some of the queso fresco and cilantro. Serve with a fresh lime wedge to squeeze over the top.
Serve while still warm.