At one point during development, the player would've been able to visit this part of Hoover Dam. However, it was cut at some point and it's map, called HooverDamIntArizonaSpillway in the GECK, deleted. Arizona Spillway was possibly an early to mid-way cut as one of the companions, Rose of Sharon Cassidy, had a dialog to say if the player visited there. Only the exterior of the massive spillway remains in the game. Regardless of its status, the spillway is specifically indicated in the official strategy guide.
The actual spillway tunnel is the big pipe/ditch area which has a bridge over it by the endgame. If you look under the bridge there is a large grating, as well as a small hatch, either of which may have lead to the deleted map.
↑Fallout: New Vegas Official Game Guide Collector's Edition p.372-376: "[3.33] Hoover Dam All of the Main Quest in Act III centers around this location, as do the Main Quests involving the visit of President Aaron Kimball. A stronghold of extreme strategic importance for centuries, this location has seen recent turmoil between major Faction too. A few years before current events, Caesar's Legion rolled in, led by ex-Mormon missionary Joshua Graham, Caesar's Legate. Not considering NCR's army to be a serious threat, Graham exercised somewhat lax control over the Legion. During the Battle of Hoover Dam, NCR Rangers and Sharpshooters from First Recon employed risky tactics against the Legion with the help of the enlisted Troopers, and despite horrific loss of life, Caesar's forces were pushed back, but not routed. The NCR has been holding the dam continuously since, and have fortified positions along the dam and up and down the west side of the Colorado River. Recently, the Legion pushed the NCR off of the east side of the river at the Battle of Willow Beach (which destroyed an NCR military camp) and the Battle of Arizona Spillway. Day to day military operations at the dam are under the command of Colonel Moore. While troopers are active here, there are also a large number of civilian contractors who are trying to keep the dam running. They are of a secondary concern to Moore, who is preparing from for an impending attack by Caesar and a visit by President Kimball." (Fallout: New Vegas Official Game Guide Collector's Edition Tour of the Mojave Wasteland)