Heat the measured water in a saucepan, add the curry paste and stir to blend thoroughly. Add the salt and bring to the boil, stirring occasionally.
Lower the heat to a slow boil and add all the other ingredients except the lychees and cucumber. Stir for 30 seconds.
Add the lychees and cucumber and cook, stirring occasionally, for 3–4 minutes. Serve in warmed bowls with steamed jasmine rice. Discard the piece of galangal or root ginger before serving.