Here - from Wikipedia:
Revenue Ruling 55-569, C.B. 1955-2, 483 says:
"A device ostensibly designed for submarine spear fishing, but capable of chambering and firing .22 caliber rimfire ammunition, is a firearm within the purview of the National Firearms Act. However, such device, if permanently attached to the speargun shaft by the manufacturer, would not be a firearm."
Note that this ruling is with regard to the National Firearms Act, and not to the 1968 Gun Control Act. (The National Firearms Act defines 'firearm' as machine guns, short barreled rifles, short barreled shotguns, and concealable firearms that are neither pistols nor revolvers.) This means that powerheads may still be under the authority of the 1968 Gun Control Act with regards to shipping them and purchase of them from licenced dealers.[