A 7-year-old boy remained in critical condition at a hospital in Patra, western Greece, on Monday after being hit on the head by a speedboat off the coast of Zakynthos.
The boy underwent neurosurgery on Sunday after suffering serious head injuries.
Doctors at the Patra University Hospital said the next two days would be decisive as regards the course of the boy's health.
The three people who had been aboard the boat which hit the boy on Sunday were arrested.
The exact circumstances of the accident were unclear.