Are there some Iris that are good for a water garden?

Yes, there are some Iris that prefer wet conditions. The Louisiana Iris, for example, are natural water or bog plants. The are native to the Southeastern US, and are commonly found along the edges of ponds. The Siberian Iris prefer wetter conditions than the Tall Bearded, but generally do not grow submerged in water, only near it or in very moist soil. Visit the Society for Louisiana Iris and the Society for Siberian Irises pages for more information.