When the WatermarkBehavior is set to either HideOnTextEntered or HideOnClick, moving the focus with the Tab key will not move it on the first press.

To work this around, handle the PreviewKeyDown event and manually move the focus to the next element when the Tab key is the e.Key property's value.

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;
		}));
	}
}