A 9-year-old Brooklyn boy drowned at a New Jersey water park during a trip with his summer camp, authorities said.

Hersch Meilich Grossman, of Boro Park, was at the Sahara Sam’s water park in West Berlin Tuesday when he went into cardiac arrest in an outdoor pool just after 7:30 p.m., according to Chief Leonard Check of the Berlin Township Police Department.

When officers arrived, lifeguards were performing CPR on Grossman, and removed him from an outdoor pool unconscious. Police, the West Berlin Fire Department and Berlin EMS continued to try to save Grossman — but to no avail, Check said in a statement.

A young boy who was at the water park with Grossman and attended his Wednesday funeral told The Post he witnessed the accident — and initially thought it may have been a practical joke.

“I said…maybe everybody should get out of the pool,” the boy recalled. “[The emergency responders] were very busy. [We saw] there was a real child lying over there. They were doing CPR.”

“I jumped back like, whoa,” he added. “I tried to help the children be calm and everything, get running, find [the boy’s] bag.”

Grossman was rushed to Virtua Hospital in Voorhees, where he was pronounced dead an hour later.

The boy had been attending a private event with his Orthodox Jewish group, Belzer Talmud Torah, Check said.

About 750 to 800 people were attending the camp event — and multiple lifeguards from Sahara Sam’s, as well as from various camps, were also present, Check added.

Sahara Sam’s is investigating the incident, a spokeswoman said in a Wednesday statement.

“We are deeply saddened by the incident and our prayers are with the guest’s family and friends at this difficult time,” the statement said. “We are completing a thorough review of the incident, which occurred during a private event, and working closely with local authorities. The safety of our guests and team members is our first priority.”

Police asked all campers and staff members to remain on the grounds of the water park, in their buses, as authorities investigated, The Yeshiva World reported. Everyone was finally released around 10:30 p.m.

The Camden County Prosecutor’s Office is also investigating.

The traditional Jewish funeral, known as a Levaya, was held in Yiddish outside the Belzer Bais Medrash at 18th Avenue and 51st Street Wednesday afternoon.

The small casket sat on two benches in the street, draped in a black cloth. A few feet in front of the casket, the boy’s mother, grandmother, and older sister sat sobbing, rocking and holding each other.

The boy’s father — who struggled to hold back sobs — and younger brother, Alexander, stood next to the rabbi.

The young boy was a son of R’ Moshe and Sheindel Grossman, part of a well-known family within the Hasidic Belzer community, sources said. The boy’s grandfather was a very popular rabbi for many years, community member Avraham Wolpin said at the funeral.

“What makes it tragic is that this was an outing, this was fun, and fun ended in tragedy,” he said. “And it also touched on a prominent family as well.”

The boy’s cousin, Joseph Langsam, said the family is turning to their faith to cope.

“We believe that this happened to be at this time and this place,” Langsam said. “We don’t feel that this was intentional in any way, of course. We don’t feel that we endangered our children by sending them to this park.”