A group of youths fell into the lake at Ashton Park in West Kirby after assaulting a man.
The incident happened at around 5.15pm on Bank Holiday Monday when the victim, who was in his 50s, was reportedly attacked and struck around the head with a bike seat.
He was taken to hospital for treatment to his injury and later discharged.
The altercation began when the man shouted at the youths for damaging metal railings near to Carpenters Lane.
The youths then approached him and assaulted him. During the attack, some of them fell into the lake before fleeing the scene.
They are described as wearing all black clothing.
The bike seat used in the assault was left at the scene and has been recovered by officers.
Detective Inspector Dave Sung from Merseyside Police said: “This was a vicious assault which left the victim with a deep cut to his head. This unprovoked attack was caused by the victim trying to do the right thing and attempting to stop the youths from causing criminal damage in the park.
“The park was busy yesterday, particularly around the lake where this incident occurred, so we are hoping that it was witnessed.
“If you saw this assault, or have any information about the incident, please come forward.
“We believe this group of youths were also involved in a disturbance at Hoylake train station earlier that same day. If you were at the train station and saw anything suspicious, please let us know.
“I would urge parents to make sure they know where their children are when they are out and about in the local community.
“The actions of children, or anyone, who commits anti-social behaviour or more serious offences cannot be tolerated and we would ask parents to play their part in ensuring their children don’t end up on the wrong side of the law.
“Furthermore, if parents are aware that their child has committed an offence, they should do the right thing and contact us.”
Anyone with information is asked to DM Merseyside Police Contact Centre or @CrimestoppersUK on 0800 555 111 quoting reference 23000459859.
