The Nautilus was failed B.O.W. that was thought to have been disposed of on the Umbrella Waste Disposal Facility, but due to the mismanagement of the facility, the creature was allowed to grow and reproduced asexually in the waterways. Observations and data were collected on the strange mutation, but its origins remain uncertain. They were originally marine ascidiacea (more commonly known as ascidians or sea squirts).
There are only two Nautilus in the entire game, both found in the Waterways, after Bruce jumped from the cruiser before its crash. The Nautilus cannot harm Bruce. It only constantly produces Torpedo Kids, but can still be killed. The Shotgun is recommended.