Combine water and wild rice in a saucepan, and bring to a boil. Reduce heat, cover, and simmer for 50 minutes, or until tender.

Preheat oven to 350 degrees F (175 degrees C).

In a large bowl, toss together the cooked rice, chicken, cream of celery soup, pimentos, onion, green beans, mayonnaise, water chestnuts, salt, and pepper. Transfer to a 3 quart casserole dish.

Bake in the preheated oven for 25 to 30 minutes, or until heated through.