I recall reading an article about someone who had tried to get close to lava for underwater pics and it recounted a hellish tale of multiple near death experiences (on a single dive). The ultra-hot rock would encounter much cooler water and the forces of uneven cooling would cause fist-sized parts of the lava (rocks, essentially) to shoot off like ballistic projectiles (steam powered, if I recall correctly). The author said he was lucky to escape with his life and would never try that again. I've never encountered anyone else that could corroberate his story. Wonder why...