When setting this property to CaretToEnd, a possible workaround which can be used is to subscribe to the GotFocus event of the control. In the event handler you can set the SelectionStart property to the length of the text using a Dispatcher.BeginInvoke() method.  

private void RadWatermarkTextBox_GotFocus(object sender, RoutedEventArgs e)
{
	var watermarkTextBox = sender as RadWatermarkTextBox;
	if(watermarkTextBox!= null)
	{	
		Dispatcher.BeginInvoke((Action)(() =>
		{
			watermarkTextBox.SelectionStart = watermarkTextBox.Text.Length;
		}));
	}
}