1.Combine honey, mustard, juice, oil, the water and garlic in a small bowl. Season to taste.
2.Brush prawns with honey mixture; cook on a heated oiled grill plate (or grill pan or barbecue), turning and brushing with mixture, for 4 minutes or until prawns are just changed in colour.
3.Meanwhile, boil, steam or microwave potatoes until tender.
4.Using a mandoline or V-slicer, thinly slice fennel and onion.
5.Place potatoes, fennel and onion in a medium bowl with watercress; toss gently to combine.
6.Combine aïoli and the extra water.
7.Serve prawns with potato mixture; drizzle with aïoli, sprinkle with dill.