its not Arizona law you look to for lake pleasant because lake pleasant is from the Colorado river the laws go to cost guard because it flows into the ocean.
Unfortunately the definition of "navigable waters" upon which USCG authority depends isn't quite as simple as the definition your instructor gave you. The USCG and the Army Corps of Engineers ("ACE") interpret "navigable waters" differently, and it is often the ACE's interpretation that gets repeated as authoritative. The definition you cite sounds a lot like the ACE interpretation. Most likely it's because construction businesses in Arizona are constantly running up against the ACE interpretation for 404 permits anytime a wash or dry riverbed needs to be modified for development. The ACE takes an extremely broad position, often expressed as "if there's a chance that I can drop a toothpick in 'X' ditch, and it might get to the ocean someday, the ditch is a navigable waterway."
33 C.F.R. Ž§2.36 defines "navigable waters" over which the USCG can exercise control. That definition is actually limited in scope and depends upon satisfaction of several conditions before a waterway located entirely within a state can be "navigable waters." To my knowledge, the USCG has only exercised jurisdiction over the Colorado River, and even then, its proscriptions have been relatively minor. I am not aware of any determination that other waterways within the state are considered "navigable waters" by the USCG. The District 11 command is supposed to be sending me a list of navigable waters within its boundaries, but I haven't seen it yet.
EDIT: Concerning who can make laws governing waterways, in 1912 when Arizona became a state, it received all navigable waters within its boundaries to hold in trust for the public. States can certainly make laws governing their trust waterways, although conflicts do arise when a state seeks to govern a "navigable water" subject to exclusive federal jurisdiction. Case in point is the Santa Cruz river, which in the last few years has been determined to be a "navigable water" for the purpose of the Clean Water Act. A dispute arose over whether Arizona could enact laws more restrictive than the federal Clean Water Act, and the key issue was whether the Santa Cruz river is a "navigable water".
Also, several appellate courts have determined that an artificial or natural obstruction that would prevent a waterway from being used as an interstate highway for commerce effectively precludes admiralty jurisdiction. The trend in admiralty is for a more restrictive interpretation of "navigable waters" except in cases involving environmental issues.
But I'm not a Proctor in Admiralty, so I could be wrong.