Don't think of snorkeling solo as not diving solo. It can be just as dangerous, especially if you are freediving. I am sure experienced free divers understand and can explain shallow water blackout. Essentially, you get to where you are awake at depth, but the partial pressure of oxygen is not sufficient to keep you conscious at a shallower depth. Thus you will pass out as you head toward the surface. With others around, your natural body reactions will close your throat, and you can simply be brought to the surface where you should spontaneously begin breathing again. When alone, you could easily suffer the same fate as Kevin Book, the former director of Scuba at the University of Florida. He was free diving alone at night at the Ginnie basin (~24 ft IIRC) and experienced shallow water blackout. Being alone and apparently negatively buoyant, it took a bit for the nearby people floating in a raft to realize that he hadn't surfaced and to then see his light on the bottom.
This is proof that diving with or without scuba gear does not suffer fools. If you break the basic rules, you take great risk, and stand a good chance of being remembered for your mistake, not the many successful cave dives you completed and the many safe divers you trained.