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 ancestor with Dragons.
Whilst carrying Temeraire and William Laurence to China in 1806 The Allegiance was attacked in the Indian(?) Ocean by the largest Sea Serpent recorded by Europeans. During the attack Temeraire instinctively felt that the serpent was a sentient creature and was disturbed by his inability to communicate with it and the eventual necessity to kill it.