If the tank ruptured and released the pressure, then that's that. However, there have been some terrible explosions occur on water heater which did not have a TP. As alluded to above: Water at atm boils at 212º. However, the pressure inside your water sytem is much higher: maybe 60 normal, and as the heater heats out of control, the pressure goes higher. Because of the pressure, the water does not boil. NOW, when something finally ruptures and the pressure goes towards zero, a lot of that water flashes to steam instantly. The energy contained therein is condiserable.
There are some photos on the internet of the aftermath of some of these accidents. I will try to find them.