Lightning Rod will look at the type of the move after Normalise is applied. Therefore, it will hit as a Normal move and Seaking will receive damage. This works for any type of Normalised move; Normalised Thunder Wave can paralyze Ground types (which normally wouldn't work because Ground types are immune to Electric type moves), and Normalised Water type moves can hit Pokemon with Water Absorb (which normally wouldn't work because Water Absorb Pokemon are immune to Water type moves). As you can see, the general rule here is that a Normalised move will always affect Pokemon that would be immune to the type of the move. Because of this, it will not affect Pokemon that are immune to the move used for any other reason. Fore example, a Normalised Toxic does not affect Steel types because they are immune to getting poisoned, not only immune to Poison type moves. You can see it for yourself here.