How difficult is it to blow up the dam such that the water rushes out? I mean bombing a place to kill people is easy, but I have no idea if you have to hit specific spots on the dam or whether it is enough to just hit any area.
The allies (the RAF) had fairly specialized bombs in WWII to blow dams. They would fly in from upstream and drop a bomb shaped like a barrel (picture a barrel of oil) from very low altitude. The bomb would skip along the surface as it lost momentum and then sink close to the dam wall. At a certain pressure it would blow and take advantage of the hydrostatic shock to break up the wall.
One of the most interesting things IMO was how they used angled lights under the planes - shone onto the surface of the reservoir to make sure they were at the correct altitude to drop the bomb (incorrect height and it wouldn't skip)
They were trying to do something completely different, with different limitations than Islamic State has. They can get a few operatives up to the dam and place the explosives, but they probably don't have access to planes or the ability to build such specialized bombs.
Those guys are crazy. Not very competent, but crazy.