Luckily not that common especially with well-maintained dams. But it was a huge fear with the Mosul dam in Iraq, which was expected to put over a million lives at risk during the ISIL insurgency: https://en.wikipedia.org/wiki/Mosul_Dam
I'm not anti-hydro at all! But it's important to understand how powerful these dams are...
I don't have data but I assume that most immediate dam breach deaths are from acute force trauma (and then drowning) rather than from just drowning. Less similar to the slow death of being without a boat in the middle of an ocean than to the hypothetical of getting stomped by the Terry Gilliam foot in Flying Circus.
Suddenly, Chernobyl doesn't sound that bad anymore.
With hydroelectric accident you drown. The end.