You know... I'm willing to bet that the Sea Serpent is Jörmungandr. In the end, Jörmungandr is another version of the Midgardsormr, but it's a sea serpent first and foremost. Being a Sea Serpent makes more sense for Jörmungandr than a Glacier Wyrm. He's also surrounding (and terrorizing) a city like in myth. I remember when I started XIV back in the original alpha... I quickly decided the Serpent wasn't Leviathan because the mouth just didn't match Leviathan. It's too barracuda like. Anyway... According to Myth, Jörmungandr was supposed to have grown so large in encircled the earth... and the Serpent is presumably larger than Leviathan. I would make the assumption Leviathan isn't much bigger than the average primal, and he could probably fit on the old boat. The Serpent is clearly larger than the boat... kinda like a certain serpent said to encircle the world, right?
Those are my predictions, anyway.