DBuss: your views of morality are your own views but you have the wrong idea regarding the law in some cases. You write:

"If you start swimming out to help me and change your mind, you can. "

Actually, that's tortuous conduct you can get sued for. If you see someone drowning you have no obligation to help, but if you start rescuing the victim you have a legal obligation to finish the job, on the theory that by starting the rescue operation you could may have convinced other bystanders to remain on the shore.