Rescue mission chief Narongsak Osotthanakorn on Monday said the operation on the second day started at 11am.
"We have sent the team in at around 11 a.m".
Authorities said the next phase could begin any time within a 10-hour window that began about 0000 GMT (7am local time) on Monday.
Xylem, which has a market capitalization of about US$12 billion, has previously helped in rescue operations after mine collapses in Pennsylvania and Chile.
But bringing out the remaining four boys and their coach could take more than one operation, Narongsak warned.
Ambulances have been moved and preparations made Monday to rescue the second batch of boys from the flooded Luang Khun Nam Nang Non cave.
"Got more great feedback from Thailand".
Musk on Sunday shared photos and videos of a small submarine-like vessel that could help rescue the group stranded on a muddy ledge about 2.5 miles from the cave's main entrance, and about half a mile below the surface.
They are being kept separate from their families at present because of infection fears, authorities said.
Officials are reportedly not telling parents their children are safe to prevent pain and anguish to the parents of boys still inside.
The boys' nightmare experience - trapped in claustrophobic darkness by rising waters - resonated across the globe, riveting people both in Thailand and internationally who anxiously watched the news coming from this town along the border with Myanmar.
So far those who have been released have been rushed off to hospital, and the most recent boys to make it out have been stretchered off.
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Authorities have said the boys are being held close to divers and wear oxygen masks to enable normal breathing.
Musk said: "Continue to be amazed by the bravery, resilience and tenacity of kids and diving team in Thailand".
Thailand's Interior Minister Anupong Paojinda said the divers who took part in Sunday's rescue were also conducting the current operation.
"We have not been told which child has been brought out ..."
After his visit, he went to Chiangrai Prachanukroh Hospital to visit the rescued boys.
But Narongsak said earlier that recent mild weather and falling water levels had created optimal conditions for an underwater evacuation.
The boys were first discovered by British divers Richard Stanton and John Volanthen on Monday. Soldiers, medics, engineers and volunteers in yellow shirts milled around but the mood was relaxed.
"I feel very happy, everybody is happy", said Hnin Jaiwong, the mother of one of the trapped boys, 13-year-old Sompong Jaiwong.
'Doctors have told us we must be careful about the food that is given to them because they are fragile after starving for many days. On top of all the concerns inside the cave, Thailand's rainy season is looming.
It is reported that the remaining four youngsters and their coach will remain in the cave for another night.
The president of soccer's governing body, FIFA, has invited the boys to the World Cup final in Moscow on Sunday if they make it out in time.