Actually, the bombs each contained nearly three tons of torpex - still a nasty explosive - and was delivered 30 feet under the water. Mohne dam took 4 of these to breach it. So, as I said, it would have to be a very big speedboat with submarine accessories...
Or a sinking houseboat?
www.sumerset.com There are around 150 or so similar to these on Lake Mead; dozens of them are rentals
www.houseboating.org/MEAD/overview.cfm
A large airplane? Maybe the type that fly over Lake Mead everyday on tourist flights from Las Vegas to sightsee over the Grand Canyon and also Lake Mead itself?
This seems to be going along the lines that everyone believes that I believe it (or any dam) would be an easy target. That's not my point at all. However I do believe it would be an EASIER target than a nuclear reator. After all, nuclear reactors are on private (or government/company) property and do not generally attract tourists. Dams however are the opposite. They create lakes which generally become public recreation areas with easy public access and hundreds of boaters, skiers, fishermen, etc on the water at any given moment.
Either target would be valuable in disrupting power supplies. A dam would also be an attractive target to destry property downstream and reduce water supplies (emptying the reservoir); whereas a reactor would have the added benefit (in the eyes of a terrorist) of spreading (or at least creating the fear of spreading) radiation.
If I seem a bit paranoid remember after retiring from the Air Force, I was a cop for 13 years. It's in my nature to see threats behind every tree (or every wave or cloud in this case) And I've been a recreational boater on Lake Mead (in the 1970s/80s) and the Gulf of Mexico for the last 20+ years as well as a private pilot/professional aircraft mechanic and hold a CDL (comercial driver's license) The point being I'm suspicious of everything and have a fair knowledge of common transportation (weapons delivery) methods.