Well, considering she can create mirages/mist to confuse the enemy ( cancels out his speed and accuracy ), she probably can! Some lightning won't do him good in any case.
That's true too, if Arlong does decide to go into water for an advantage in speed and power, Nami can thunder shock the water xD I would pay mighty to see that fight ^^
But here's a question: If Nami were to fight Arlong in her present state (Post timeskip), with her arsenal of weather weapons, would she win?