Score the back of the prawns with a sharp knife to butterfly them. It'll help them soak up the marinade. In a large bowl, mix the mayonnaise, lime juice, garlic and red curry paste to create the marinade. Stir in the mint and coriander, add the prawns and coat thoroughly.
Set aside in the fridge for at least 30 minutes. Meanwhile, make the salsa by chopping the watermelon and cucumber into 1cm cubes and mix with the shallots, mint, chillies and lime juice in a large bowl. Set aside for the flavours to blend.
Thread the prawns onto pre-soaked wooden skewers and BBQ for 2-3 minutes each side, or until the prawns are pink and cooked all the way through.
Serve the prawns with the salsa on the side, jasmine rice and salad.