Sea Serpents can be found in all major oceans and seas. The sizes and presumed breeds of Sea Serpent varies across the globe, but their general body structure argues for a common evolutionary ancester with Dragons.
The Allegiance was attacked in the Pacific(?) Ocean by the largest Sea Serpent recorded by Europeans. During the attack Temeraire instinctivly felt that the serpent was a sentient creature and was disturbed by his inabillity to communicate with it and the eventual necessity to kill it.