You wouldn't need nukes or asteriod strikes.
If you forget about trying to make it into a shipping canal, you could simply build a tunnel/canal and then let gravity do the rest... You would create a man made river that flowed from the pacific ocean to Death Valley. Evaporation would continually pull water from the ocean, you could control the flow with locks and valves.
You wouldn't really need to flood the entire valley either!
But it would be fun to cut a shipping canal through mountains. While your at it, you might as well continue the canal all the way to the Atlantic!