Transient global amnesia is a well defined medical condition (although still not fully understood) and I would suspect life threatening if occurring underwater. Even though people affected by it don't become physically disabled in any way, the loss of short term memory is absolute; they can't remember where they are, where they are going or what they are doing, repeating the same questions every 5-10 seconds over and over again. Because of this they are usually unable to plan or execute any actions that take more than a couple of seconds. They very often become panicked and their behavior can become irrational. Finding yourself in an alien, immediately life-threatening environment and being unable to escape it unaided sounds like a perfect recipe for disaster to me. Keeping a fistful of plastic in their mouth isn't something most people would be instinctively inclined to do either. My view on this is biased as I see these people basically on a weekly basis, but I don't feel like it is all that rare a condition. In fact I have been somewhat surprised how this condition is not better documented in divers. Then again, there are a number of unexplained fatalities due to rapid ascent or drowning, so who knows...
The symptoms of TGA typically last about 3-6 hours and can last up to a day, but practically never less than an hour. I believe you have experienced amnesia due to nitrogen narcosis, which is easy enough to avoid in the future