Wolverhampton Crown Court found Alex Cotterill, 28, guilty of breaching his duty to assure the health and safety of pool users at the University of Wolverhampton's Walsall campus.
Father-of-three Adrian Miles, 48, had gone swimming at the university pool as part of his recovery from a rugby injury in July 2006, but got into difficulties and was later found dead at the bottom of the pool.
The Health and Safety Executive (HSE) brought the prosecution against Cotterill, who refused the charges.
Speaking of Cotterill's conviction, Kanwal Kanda, HSE Inspector, said: "It is completely unacceptable for those with a responsibility for the safety of others to neglect their duty.
"While the lifeguard did not cause Mr Miles to get into difficulty, his neglect of duty was clearly a major factor in leaving him and potentially others using the pool unnecessarily exposed to risk.
"Standards are well established through training and they must be applied by lifeguards to ensure swimmers are safe."
The jury could not reach a verdict on a second lifeguard charged with the same offence and he was discharged.
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