Stir the quinoa and water together in a microwave-safe bowl. Cover; cook in the microwave on high until the water has been absorbed and the quinoa is tender, about 9 minutes. Uncover and place into the refrigerator to cool.
Place the celery, onion and mayonnaise together in a bowl. Season with wasabi, lemon juice, salt and pepper; stir to mix. Fold in the tuna and chilled quinoa until evenly blended.