The amount of explosives needed to "take out" Hoover Dam would require multiple cargo trucks. And I'm not sure security would just turn their heads at multiple cargo trucks driving atop the dam (especially since traffic is now routed across the Pat Tillman Bridge).

I guess you may be able to fly an airplane into the dam from the south, but good look maneuvering an airliner through that canyon and even still, an airliner probably would only dent that dam.