Bring the water to the boil in a saucepan, add the rice, cover and simmer gently for 20–25 minutes until very soft. Drain off any excess water. Mix the vinegar and wasabi and stir into the rice. Leave to cool. Toss the prawns with the lime rind and juice and set aside.
Place one nori sheet on a bamboo rolling mat, spoon over one-fifth of the rice and spread into an even layer, leaving a small border of nori showing. Arrange one-fifth of the smoked salmon in a long line in the centre of the rice. Next to that add one-fifth of the prawns, and on top some ginger, pimentos and spring onions.
Using the mat to help, roll up the nori sheet so the rice wraps around the filling and the edges of nori overlap slightly. Rock back and forth for an even shape. Repeat to make the remaining rolls. Wrap them individually in clingfilm and chill until required.
When ready to serve, cut the rolls into thick slices and arrange with cut edges uppermost on a plate. Serve with extra wasabi.