you mean ammonium nitrate don't you? Urea is neither an explosive nor an oxidizer. Second if you blew up your boat boat on the top of the dam, then you'd have a loud noise and the dam might lose some guard rails - the dam would be unhurt. Blowing up a dam is a very difficult proposition becasue they are very large tough structures - this was tried during WWII and it took very special bombs and delivery systems to damage a dam.
This is just another load of alarmist crap from the government to scare the bedwetting public into giving up more freedom.
It took a special delivery system because the other side was shooting back!
BTW, the big secret was set the bomb off close to the base of the dam. This is a piece of cake when you can simply motor up to the face of the damn dam and scuttle your boat.
Heck, since you don't have to get the charge into a light enough package to airlift, you can "use enough dynamite, there Butch"...