The 58-year-old manager of the park and his 30-year-old son jumped into the water to save three flailing children, who were caught in an electrical current.
Rescuers attempted resuscitation but were unsuccessful.
All five were taken to hospital after other park personnel cut power to the pool, but their hearts had already stopped, the hospital told the Daily Sabah newspaper.
Two of the victims were in their teens, and one was 12 years old.
Two other people were reportedly injured when they touched metal railings near the pool.
The park was reportedly undergoing renovations and many electrical cables were around the pool, one of which could have fallen into the water.
Hasan Akcan, the mayor of Akyazi where the park is located, said the incident was “saddening.” He added that the park had received a license in 2012 as a swimming pool and amusement center, according to state-run news agency Anadolu.
It was not immediately clear what caused the tragedy, but Anadolu reported that a probe had been launched.
News agency Dogan reported that the park lacked a residual current circuit breaker that shuts down power to prevent electrical shocks.
aw/cmk (AFP, AP, Reuters, dpa)