I've had it happen twice to me. Kicker is it wasn't either as a DM and wasn't my buddy or group. Was also in a rock quarry... Fun stuff let me tell you. Many years of lifeguarding and the training that went with that was much more helpful than what we were taught in scuba. If you train your students well, that type of stuff should present itself in the pool long before you make it to open water
You've been killed two times by your students? Dam that must hurt, at least the first time anyway. :shocked2: