The small yellow submarine cruising around the Mandeville harbor looked like a brightly colored toy as St. Tammany Parish Sheriff’s Office deputies stationed on the dock fed out its tether and peered at a computer screen that showed sonar and video views of the lake bottom.

The morning demonstration revealed nothing more than an old tire. But the VideoRay, a remotely operated vehicle that has sonar, a camera, a light source and an articulated arm, is normally looking for something far more important.

Earlier this month, the device helped divers search for the body of 38-year-old Tam Van Nguyen, a Biloxi, Mississippi, man who had jumped into the lake from the Interstate 10 twin spans.

The dive team used the submarine to search around the bridge’s pylons. Sand in such areas tends to wash out, creating deep holes that can’t be explored by the side-scan sonar that the Sheriff’s Office uses in grid searches of larger areas, said Sgt. Scott Lee, a member of the dive team.

The VideoRay is designed for just such a scenario. Instead of having to send divers down to check out the holes, the Sheriff’s Office could let the little submarine do the work, ruling them out as possible recovery sites.

That helps the dive team meet what Lt. Bret Ibert of the Sheriff’s Office described as a key goal: recovering bodies as quickly as possible to provide closure for families and allow for the possibility of an open-casket funeral.

The VideoRay and its computer hardware and software arrived about two months ago and cost the Sheriff’s Office $80,000. It’s the newest tool that the 10-member dive team uses to find targets, whether they are looking for a piece of evidence, such as a gun, or the body of someone who has drowned.

The ability to search beneath the water is critical for St. Tammany Parish, which has 279 square miles of waterways, including rivers and bayous as well as Lake Pontchartrain.

The underwater environment is a dim, murky place where divers often rely far more on touch than on sight. That kind of work is known as black water diving, and it’s dangerous as well as difficult, according to Cpl. Kenny Kustenmacher, who has been diving for 22 years and has commanded the dive team for 12 years.

“It’s all feel down there,’’ he said.

Divers might find rebar, spikes, jagged metal and other objects from overturned docks, storm debris or sunken boats — all with the potential to cause injuries.

Hazards can be natural, too. Kustenmacher was hit in the finger by a stingray when he grabbed an anchor rope and was working his way along it. The sting meant a trip to the hospital and a week of lost work.

Divers sometimes encounter dangerous substances, too. Fuel or battery acid leaking from a submerged car or chemicals from a drum can burn exposed skin.

Technology helps the Sheriff’s Office to be more selective in deciding when to send down divers, saving time and energy and speeding up the search. Divers also can be more prepared for hazards if they know what they’re likely to find — for instance, donning protective gear like helmets or Kevlar-lined gloves.

The technology doesn’t eliminate the danger of black water diving, Lee said, but it helps mitigate the risks.

“You don’t go barreling in,’’ Kustenmacher said. “You might go in from a different angle.’’

Being able to look underwater from above can eliminate the need for a dive, he said, either by confirming that what the dive team is seeking isn’t there or by determining that the object can be recovered with the articulated arm.

“All roads lead to the diver,’’ Sheriff Jack Strain said. “It’s a matter of how quickly and safely we can let the diver do his job.”

Advanced technology isn’t new to the dive team. It has been using side-scan sonar for about five years. That device, which can be moved from boat to boat, has a range of 100 feet on both sides and is used by the dive team to search wider areas. In July, the team used side-scan sonar to locate the body of Darrel Brown, a former Nicholls State football player who drowned when his sailboat flipped in the Tchefuncte River.

In March, the St. Tammany Sheriff’s Office assisted the New Orleans Police Department in the search for 19-year-old Hayley Howard, a University of New Orleans student who disappeared on her drive home to Slidell.

Howard’s car had gone into an Irish Bayou waterway, but helicopters couldn’t locate it because it was under a bridge. The dive team used side-scan sonar to eliminate waterways one by one and ultimately spotted what turned out to be her car. NOPD divers entered the water to make the final confirmation and recovery.

The dive team is often called upon to assist other agencies, and team leaders say the St. Tammany unit is more prepared than others to deal with searches like the one conducted for the missing student.

Now, with the arrival of the VideoRay, the Sheriff’s Office will be able to use both devices in concert, Capt. George Bonnett said. The side-scan sonar will perform grid searches and identify objects of interest for the VideoRay to look at more closely.

The devices also will allow searchers to mark a location with GPS coordinates, something that the dive team used to do by sinking a marker, using an old broken wheel rim from a police car attached by chains to rope.

The VideoRay has a maximum depth of 1,000 feet, although Kustenmacher said the deepest hole he’s had to dive in is only 127 feet in the Rigolets.

Technological advances don’t replace the human element. Divers still play a critical role, using their experience to search by touch.

Lee equated that process to walking through a dark bedroom in the middle of the night: You know where the light switches are and can locate one by feel. “Using your mind’s eye, you tend to recognize that,’’ he said.

Deputies can’t even try out for the dive team unless they have been qualified in basic diving. Afterward, they have monthly training sessions and annual training trips to Florida, where they can conduct exercises like recovering a car in water with better visibility.

Attaching air bags to vehicles to bring them up can be dangerous, Ibert said, and it’s easier to train in water where the instructor can see whether a diver is doing something correctly or not.

“We haven’t been able to find anyone who would let us sink a car in their swimming pool,” he joked.

But in real-life situations, divers still must rely on their sense of touch. “It’s amazing. You can tell exactly what you have just by bumping into it,’’ Kustenmacher said. “A deputy bumps into a gun and immediately knows.’’

Divers practice with items like sunglasses, a bottle or a watch. “Even with big thick gloves, you can tell,’’ he said.

Technology enhances the dive team’s ability to rule things out without having to go underwater in person, Lee said.

“It’s a tool, not a solve-all, end-all,’’ he said.

A safer and faster conclusion to each search is the goal.

“If you’ve ever seen a family on the bank, it’s just excruciating,’’ Strain said. “You put yourself in their shoes for one minute, and it’s worth it.’’

Follow Sara Pagones on Twitter, @spagonesadvocat.