Score the back of the prawns with a sharp knife to butterfly them - this will help them soak up the marinade.
In a large bowl, mix the mayonnaise, half of the lime juice, garlic and red curry paste to create the marinade.
Stir in 1 tablespoon fresh mint and the 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, the rest of the 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 barbecue 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.