Some safety net.

A stranded jet-skier seeking help effortlessly overcame the Port Authority’s $100 million, supposedly state-of-the-art security system at JFK Airport — walking undetected across two runways and into a terminal, The Post has learned.

Motion sensors and closed-circuit cameras of the Perimeter Intrusion Detection System, or PIDS, were no match for Daniel Casillo, 31, of Howard Beach, who easily breached the system meant to safeguard against terrorists.

Casillo’s adventure began at a Rosedale watering hole, where he was hanging out with friends when they decided to go out racing their watercraft.

“They were trying to see who had the fastest jet skis, like idiots,” said his girlfriend, Deanna Cowan.

But Casillo’s ride broke down in the dark waters of Jamaica Bay at around 7:45 p.m. — and his pals didn’t notice they had left Casillo behind.

With his craft taking on water, he called Cowan in a panic.

“He said, ‘I’m stuck!’ and told me to call his friend Albert to come out and tow him in,” Cowan, 28, recalled.

But help didn’t come, and the stranded Casillo swam three miles toward the only thing he could see — the lights of Runway 4-Left, which sticks out into the bay.

“He didn’t want to go that way, because you’re not allowed to go over there,” Cowan told The Post.

Dripping wet and in a bright-yellow life jacket, Casillo climbed the perimeter fence, which is eight feet high, and walked across that runway and intersecting Runway 31L — and made it all the way to Terminal 3 without anyone noticing.

Casillo was finally apprehended when he approached a Delta Airlines worker near Gate10. He was charged with criminal trespass.

“All his IDs, his money, his car keys — they’re all gone. They sunk with the jet ski,” Cowan said.

The highly touted PIDS system, which was first announced in 2009, has proven to be a boondoggle, having never fully come online after a series of troubling delays.

“The Port Authority PBA has been concerned about the failure of the PIDS system for quite some time. We have brought this to the attention of former Executive Director Chris Ward, who failed to act,” Robert Egbert, spokesman for the PA PBA.

The PA, which runs the airport, says that it has stepped up patrols and is reviewing the incident and that it plans to meet with PIDS maker Raytheon this week about the breach.