Remove the seeds from the cucumber using a teaspoon and slice it thinly. Add the salt and toss to coat. Leave the cucumber to stand for 5 minutes, then squeeze out any excess water.
Soak the wakame in a small bowl of water for about 5 minutes, then drain.
Bring a small saucepan of water to the boil over medium heat. Add the prawns and boil for 2 minutes. Drain and leave to cool.