In France at least, and I believe in the US to, it is illegal to not do something if you can.

It does not mean that you should dive and bring him back. In fact, it is not recommended unless you know what you are doing as you may put yourself in danger and need rescuing yourself. But if there are other people around who can help and you don't alert them, or if you have a working phone and don't call whatever emergency number is appropriate, than that's illegal.

EDIT: It appears that it is not illegal do do nothing in most of the US. The law only protects you from consequences of trying to help.

It depends what you mean by do. In the US, if you didn't notify police or call for help and just stood and watched while someone died, no jury would pass on convicting you. You're expected to behave reasonably. There need not be a written law. It's called common law.