Put the shrimp in a large resealable plastic bag. Add the oregano, crushed red pepper, grated garlic, lemon zest and juice and 2 tablespoons of the olive oil. Seal the bag tightly and use your hands to lightly massage the marinade into the shrimp. Marinate for 15 minutes at room temperature.
Prepare a grill or grill pan for high heat; lightly oil the grill grates or pan. Soak the skewers in cold water.
Put the watermelon and cucumber in a mixing bowl. Put the feta and olives on top. If using the mint, scatter most of it over the watermelon mixture. Do not mix.
Thread about 5 shrimp on each skewer. Halve the baguette piece lengthwise, then cut the pieces in half crosswise so you have 4 pieces. Brush the cut sides of the baguette with 2 tablespoons of the oil and sprinkle with salt and pepper. Put the bread cut-side down on the grill along with the shrimp. Grill the shrimp until lightly charred in spots on both sides and just cooked through, 1 to 2 minutes per side. Grill the bread until lightly charred, about 2 minutes. Remove the shrimp and bread to a plate.
Lightly rub the cut sides of the halved garlic onto the hot bread. Put 1 piece of bread on each of 4 serving plates.
Add the remaining 1/4 cup olive oil and a large pinch of salt and pepper to the salad, gently stir to combine and divide among the plates. Top each serving of salad with a shrimp skewer. Garnish with the remaining mint if using and serve immediately.
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