How much explosives do you need to destroy a Soviet reinforced dam like this? Several tons at least, perfectly placed. These mammoths were built to withstand American nukes.
If the Ukrainians did it, they were able to smuggle a fantastic amount of explosives under the Russians' noses into Russian-controlled territory and then blow it up, without any cars or mechanization. Impressive ninjas.
With underwater explosion in the direct proximity you do not need that much, especially if you will use a charge with explosively formed penetrator. 16 meters of water pressure will quickly exploit even a small hole. My guess, it's small enough to be deliverable by a remotely controlled boat like those used by Ukraine to attack the Russian fleet, naval bases and the Crimean bridge.
If the Ukrainians did it, they were able to smuggle a fantastic amount of explosives under the Russians' noses into Russian-controlled territory and then blow it up, without any cars or mechanization. Impressive ninjas.