Police found the body of a man in water in a wooded area near the Goodwyns housing estate in Dorking at about 04:20 BST.
Searches are under way and officers are still working to identify the man who died, Det Ch Insp Paul Rymarz said.
A 21-year-old man is in custody. Anyone with information is urged to contact Surrey Police.
A man has been arrested on suspicion of murder after a body was found in a stream in woodland in Surrey.