I used to live in Vegas many years ago, was a truck mechanic, had to go work on trucks stuck on the switchbacks coming and going to the bridge, its a real bottleneck, and yes its also a very easy target for a terrorist.
All one would need is a type of trailer that could have a track that would drop several underwater mines over the water side of the dam spaced apart and on consecutive timers.
Dambusters by an 18 wheeler. A very determined intelligent radical can do it, trick is to research at what depth is optimum and at what point they need to be dropped.
I rode across the dam on my motorcycle about 2 years ago and was waved past a road block about a mile before the dam.
Everyone in a vehicle larger than a motorcycle was being pulled over and searched....presumably as a precaution against any such explosive exploit.
FWIW, the drop from the bottom of the Hoover Dam to the Pacific Ocean, the total drop in the Colorado river from the base of the dam to the sea is only about 300 feet over a run of several hundred miles. In New England rivers often drop five feet per mile (Assebet, Blackstone, ...). By New England standards, the Colorado from Boulder to the Pacific is a pond.