The amount of water 'released' in this story would release enough energy to melt the Earth.
However, silly stories aside, the effects of water on top of magma depends where this occurs. Along oceanic ridges the pressure at those depths keeps things rather muted. The lava that is released is flash cooled creating what is known as pillow lava. When this happens at the surface with little pressure to keep the energy in check the effects are going to get explosive with the water turning to steam and the surface of the lava solidifying before being explosively destroyed by the still molten lava underneath. You would not want to get too close.
For your question specifically, consider that the highest active volcano is 6893 m high whereas Everest is 8848 m high. With that in mind this flood would have covered that volcano with almost 2 km of water creating enough pressure to have calmed things down sufficiently that you could have focused better on the fact the entire Earth had melted...