He had been part of a team of three commercial divers tasked to do welding work on the underside of a docked container ship on Wednesday evening.
But Mr Edmond Kwok, 36, was killed when a suction valve of the porthole that he was in was turned on suddenly, sucking him in with great force.
Mr Kwok had been taking photographs of welding work inside the porthole of the ship at Marina South Pier when it happened.
The suction force, which varies depending on the type of ship, is believed to be what killed Mr Kwok.
The porthole, known as a sea chest, is meant to suck in seawater to cool the engines and generators on board. If switched off, the ship would not be able to power its systems, lights, air-conditioners and other components that require electricity.
Two other divers, who were with Mr Kwok at the time, tried to save him. But they were unsuccessful and could only watch him get pulled away by the force, Mr Kwok's younger brother told Chinese newspaper Shin Min Daily News.
When contacted, a police spokesman said Mr Kwok was pronounced dead at the scene and they are investigating it as a case of unnatural death.
The New Paper understands that his body had been brought out of the water when the police arrived.
The incident happened at around 6pm, when the sun was setting.
An expert, who declined to be named, said large container ships would have several sea chests at different locations. Crews of these ships could have continued to run the electrical generators by turning on sea chests which are further away from where underwater work was being done.
These sea chests can only be switched on from the generator room or the control room of the ship, he said.
Commercial Diving Association (Singapore) chairman Abdul Malik said that whenever a diving company arrives at a client's ship, the diving supervisor must first meet the ship captain and agree on the work being done, and also on the standard operating procedures.
These procedures, which are all set out in a technical advisory for commercial diving safety, include preventing the engine from being turned on, putting a physical padlock over the switches and putting up warning signs that say the systems should not be switched on due to an ongoing dive operation, he added.
"These systems include all propellers, suction valves and thrusters on the ship that can harm the diver. The procedure is known as isolation, lock out and tag out, and is to be adhered to by everyone in the industry," he said.