You have to use a non-toxic shot for waterfowl. Moreover, it must have received approval for waterfowl use by the USFWS, as non-toxic. This means that you can shoot steel shot for waterfowl, along with several other types of shot (such as bismuth) and all of the tungsten based shot (hevi-shot, hevi-steel, Tungsten Matrix, tungsten-iron, TNT, TINT, Wingmaster, Extended Range, Tungsten Super Shot etc etc). There is a big list at USFWS website.

The majority of waterfowl hunters use steel, because it is much less expensive than tungsten. Non-toxic waterfowl shot is often generically referred to as "steel" which is a misnomer... much as most of the high density shots being made are generically referred to as "hevi-shot" which is a specific blend of metals proprietary to Envrionmetal, Inc of Sweet Home, Oregon.