Cut the cucumber in half lengthways, then use a teaspoon to remove the seeds. Use the thin slicer blade on the side of a cheese grater to finely slice the cucumber and carrots (alternatively very finely slice them with a knife).
Place in a bowl, add the coriander, lime juice, fish sauce and sugar then mix well. Leave to marinate for at least 10 mins.
Preheat the grill to high. Line a grill rack with foil. Season the mackerel fillets on both sides then place skin side up on the rack. Grill for 6-8mins or until the skin is crisp and the flesh flakes easily. Divide the salad between four plates, place 2 fillets on top of each and decorate with watercress. Serve with new potatoes if liked.