Police were called to Drift Service Reservoir near Penzance at 4pm on Thursday 18 February following reports that a body had been seen in the water.
Emergency services attended the scene and recovered the body of a man.
CPR was commenced by police and the ambulance service and the man was found to have sustained a suspected stab wound to his chest.
The man, aged in his 30s, was confirmed deceased at the scene. Enquiries remain ongoing to identify and locate next of kin.
A number of enquiries were carried out overnight in relation to this matter both in Cornwall and in Warwickshire.
A 50-year-old man with links to Cornwall has been arrested on suspicion of murder and remains in police custody at this time.
Detective Inspector Steve Hambly, from the Major Crime Investigation Team, said: “We’re in the early stages of our investigation to establish the circumstances that led to the death of this man.
“Scene guards remain in place at locations in St Ives and Penzance whilst officers continue enquiries, and these are expected to remain in place over the weekend.
“I’m keen to hear from any members of the public who may have witnessed any suspicious behaviour in the area of Drift Reservoir or Lelant through the period between Monday 15 February and Thursday 18 February.
“Anyone with any information which may assist the investigation is asked to contact police on 101 quoting log 686 18/02/21.”