Mix together the mayonnaise and yogurt, then stir in 1 tablespoon of the lime juice and the wasabi paste, to taste. Add the coriander and crab meat, season to taste and toss to mix well.
Cut the avocados in half lengthways and remove the stones, then brush with the remaining lime juice. Place 2 halves on each serving plate.
Fill the avocados with the crab mixture and garnish with watercress. Serve immediately.