You’re so much more virtuous than everyone else in your own mind.
Do you really consider it THAT big a deal making such a small effort to potentially save another person's life? It's easy to say after the fact, when the shark didn't actually attack, after it had creeped up on the woman, that there was "nothing to worry about". It's a common excuse for people who rarely if ever bother helping other people out.
Like seeing someone blindly walking out into traffic and (luckily) not getting struck by a car, "See, I told you there was nothing to worry about"! "And you wanted me to get off my fat lazy butt and warn them about the cars and trucks they clearly didn't see coming.