500MJ is enough energy to flash-boil 200Kg of water, so that might work. Would be a sizeable steam explosion, as the 330 or so cubic meters of instantaneously created steam tries to go somewhere, and an interesting pressure wave in the water tank.
"interesting" -- the concussive overpressure would be brutal in water and travel further I guess, maybe you could add a bubble stream to the water like in dives to diffuse the shockwave some.
