Cut the watermelon into 3/4- to 1-inch-thick slices; trim off the rind. Using a 3-inch square or round cutter, cut 10 to 12 squares or circles out of the watermelon slices. Brush the pieces on both sides with the olive oil.
Grill the watermelon, covered, until charred, about 1 minute per side. Transfer to a shallow baking pan; refrigerate 30 minutes. Drain any liquid that has gathered around the watermelon.
To make stacks, place the feta slices on half of the watermelon pieces; top with the remaining watermelon pieces. Drizzle the stacks with honey, top with the basil, and sprinkle with the salt. Serve with lime wedges.