In Southern California, I would say that freedive spearfishing alone is the norm rather than the exception. Our most desirable fish, the white sea bass, is rather sensitive to noise and movement, and most of us don't want another diver next to us scaring the fish away.
Of course it depends on your definition of "alone." I have one to three friends along on the boat and we are all in the water at the same time, but we don't try to stay near each other.
But at least on my boat, the situation changes after someone shoots a fish. Our fish tend to head for the bottom and wrap up in the kelp, and it can be very dangerous down there, expecially if visibility is limited and the diver is at the extremes of his depth capability, trying to sort out kelp and shooting line and cut the correct one. I urge everyone on the boat to call for help if he feels the least bit threatened by the situation.
Just last week I had given up on a kelp bed and was heading for the boat when I saw a buddy waving at me. I went over there and helped him out. As soon as we recovered his fish and he was heading for the boat, my other buddy was waving for help. I had to struggle through a thick mat of surface kelp to reach him, and then found that his reel line was strung out through the kelp bed and his gun was floating at least 100 feet away from where I joined him. It took quite a while to find his fish, kill it and bring it up, detach the shooting line from the reel line, then go get the gun and wind all the line back on the reel. Frankly, I would rather have spent all this time trying to find a fish of my own, but then I'd rather come home without a fish than come home and try to get up the guts to call a wife or mother and tell her that her loved one was lost. I've done that once, and once was enough. Also, I expect my dive buddies to spend their time helping me when I need it, so it all works out.
