When Nami realises that lightning doesn't affect Luffy because he's made of rubber, she asks herself if Luffy could be Enel's one and only ... NATURAL ENEMY.
It's a point that comes back again in Dressrosa Arc when it talked about Celestial Dragons and the "family of D." being their natural enemy.
Since Enel and Celestial Dragons are both referred to as "gods" by some characters, I guess Oda intentionally made Nami say those exact words to reinforce the parallel between Enel and Celestial Dragons in addition to hint that it is Luffy's destiny to oppose the Celestial Dragons, or anyone being referred to as "gods".
It's a point that comes back again in Dressrosa Arc when it talked about Celestial Dragons and the "family of D." being their natural enemy.
Since Enel and Celestial Dragons are both referred to as "gods" by some characters, I guess Oda intentionally made Nami say those exact words to reinforce the parallel between Enel and Celestial Dragons in addition to hint that it is Luffy's destiny to oppose the Celestial Dragons, or anyone being referred to as "gods".