Preheat a grill to high. Peel 1 large jicama and slice crosswise, 1/3 inch thick. In a bowl, toss the jicama with 2 tablespoons vegetable oil and grill, turning once, until grill marks appear, about 8 minutes. Let cool. Chop and transfer to a bowl. Add 1 cup chopped watermelon, 1/4 cup finely chopped mint and 1 small seeded and finely chopped jalapeno chile. In a small bowl, combine 1 tablespoon apple cider vinegar and 1 teaspoon honey. Stir into the salsa and season with salt; refrigerate for 1 hour.