variable names cannot have parenthesis in them, so get rid of the parenthesis:
public Transform leftHandController;
public Transform rightHandController;
private Rigidbody playerRigidbody;
Next, your code still won't compile because you're using a variable that doesn't exist:
Vector3 swimDirection = ( leftHandControllerposition - rightHandController.position).normalized;
should be:
Vector3 swimDirection = ( leftHandController.position - rightHandController.position).normalized;
As for the "failed to find entry points", I don't have Unity installed, so I can't test and comment on that.
It is ALWAYS a good idea to copy and paste the exact error message from the Output window into your questions and responses.