What can we learn from this?
That life will kill you in unexpected ways.
This is sort of like a guy who makes his living on a tight-rope and a trapeze getting himself killed tripping over a crack in a sidewalk.
Seriously, swimming with a bunch of sting rays is probably one of the safest things that he has ever done on his show. I always figured that an angry croc or a venomous snake would get him. But to be done in by a creature that is usually considered to be docile is one of those things that you probably can't plan for.
He will be missed.