Disassemble oranges into segments; leave 1/4 cup of juice for dressing.
Combine the dressing ingredients with 1/4 cup orange juice.
Cut the salmon into very thin plates; add half of the dressing; leave to marinate for 5 minutes.
Transfer the marinated salmon with the remaining half of the dressing to a bowl, add the orange segments, watercress; mix gently. Add spices to taste (salt, pepper).