You might enjoy reading the book Krakatoa: The Day the World Exploded, by Simon Winchester. He describes the geologic and social after effects of what happens when water enters a volcano. It's more than “steam.” It's “cataclysmic explosions. read more
If you poored water into an active volcano with lava flowing. Something like this video would most likely occure. This happens Because the water vaporizes rapidly, and steam has a much greater volume than water. Therfore it seems to explode, but the gas expansion just throws the surrounding material in all directions. read more