Colin, glad you're still with us... by the way, if you concentrate and mentally visualize the dive over and over, particularly the last seconds, then eventually you may be able to remember everything, but it could take weeks for the memories to appear. In particular, although you can't remember anything from 30ft, it is very likely that you made it to the surface still conscious, as the memory lapse goes back many seconds.
Boat exhaust can be fatal as Sean mentioned, I had it happen to me before.
To say that there is no warning is still sometimes incorrect. In Colin's case, his memory ends at 30ft. However, what could have occurred is that as he ascended to 15ft or 10ft, he may have felt very hypoxic and suddenly realized a blackout was imminent. However, since that region is not in memory (yet), what memory remains makes it appear as though it happened out of the blue.
In my case, my first ocean blackout seemed 'out of the blue', and upon awakening I was amazed that I had blacked out. However, after that and a few more sambas/BOs (under controlled conditions), I began to develop a better ability to feel the problem coming. That is not to say that such a sense is reliable, but certainly in my case I was able to develop it, but only after many blackouts in controlled conditions. For that reason, having deliberate blackouts in a course such as PFI can actually be beneficial to improve your sense of imminent hypoxia.
Spearfishing or concentrating on some task takes your mind away from subtle inner sensations, and any feeling of hypoxia may go unnoticed. Another reason why I consider 'pure' freediving safer than a concentrated activity like hunting.
