In a small saucepan over medium heat, add the water and the sugar. Cook over low heat until the sugar is dissolved. Pour 1 cup of the syrup into a small bowl and set aside.
To the remaining syrup, add the red pepper, jalapenos, garlic, and cilantro and simmer on medium heat for 30 minutes, or until thickened and marmalade-like.
In the meantime, toss the prawns in a bowl with half of the olive oil, juice of 1 lime, cayenne, chili powder, salt, and pepper. Place on the hot grill and grill each side until cooked through, 4 to 5 minutes. Set aside.
Place the watermelon on a plate and brush each side with the reserved sugar syrup and the remaining olive oil. Place on the grill and cook on both sides, about 2 minutes.
Once the red pepper marmalade is thickened, place watermelon on the plate with prawns on top. Squeeze the remaining lime over it and spoon on the marmalade. Garnish with coconut and scallions.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.