I started a new game on creative mode using snapshot 14w31a. I flew around until I found an ocean monument. I switched to survival mode and used the internal give command to grant myself diamond helmet, diamond chestplate, diamond leggings, diamond boots, potion of night vision (8 min duration), and a potion of underwater breathing (8 min duration).

I equipped the diamond armor, drank the night vision potion and underwater breathing potion and started to swim toward the monument.

As I approached, a guardian mob engaged me in combat. I saw the animation of the beam directed toward me but before I took damage, the game crashed.

Attached is the crash report.

I reloaded the world to see if I could duplicate the problem and i received damage without the game crashing. I kept on swimming around and the game crashed again right as a sound started to play (I think it is the Elder Guardian noise, but I am not sure...

Attached is the second crash report.