1.Make lime aïoli. Blend or process egg yolks, mustard, rind, juice and garlic until combined. With motor operating, gradually add oil, blending until aïoli thickens. With motor operating, add enough of the water (if any) to achieve desired consistency.
2.Peel grapefruits; separate the segments. Shell and devein prawns, leaving tails intact.
3.Combine grapefruit and prawns in large serving bowl with remaining ingredients. Serve with lime aïoli.