Bring water and chicken broth to a simmer in a 2- to 3-quart saucepan. Add chicken and simmer, uncovered, 6 minutes. Remove from heat, cover pan, then let stand until chicken is cooked through (about 15 minutes). Move chicken to a plate, let cool 10 minutes, then chop into 1/2-inch pieces. While chicken is cooling, whisk together mayonnaise, yogurt, curry, lime juice, honey, ginger, salt, and pepper in a large bowl. Add chicken, onion, mango, grapes, and cashews; gently stir to combine.