So while the Mandarin and The Ghost are possibilities, we think there might be another possibility. As already mentioned, Marvel Studios is shooting in a coastal town in a facility that boasts a special effects water tank as one of its selling point. The location lends itself to a water-based villain, which got us thinking.
In Iron Man 2, there were several easter eggs involving Atlantis, which led to lots of speculation that Prince Namor, the Sub-Mariner would be appearing in an upcoming Marvel Studios film. There have also been reports that a giant sea creature called Leviathan will make an appearance in The Avengers, which has led to speculation about Atlantis and Prince Namor, the Sub-Mariner in that movie as well.
Of course, this is all just speculation, but if this is the closest thing to a Namor movie, I'm down.