The navy also said handprints were found on the cave's walls.
The boys, aged 11-15, are believed to have entered the Tham Luang Nang Non cave in Chiang Rai province with their 25-year-old coach late Saturday afternoon.
An official at the forest park in Chiang Rai province where the cave is located said the getting to the large chamber where the students might be is difficult.
The passage to the chamber is extremely small, flooded and "covered with sand and mud", Kamolchai Kotcha said.
Further attempts later on Monday also failed to locate the boys, though rescue teams have found "traces" of them inside the cave, police in Mae Sai district said.
"Right now, our family is hoping that the children trapped inside will have formed a group and are safe and waiting for officials to go in and save them in time. That's what I'm hoping," said Noppadol Kantawong, the father of one of the missing boys.
Footage on Thai television showed bicycles and backpacks left outside the entrance to the cave.
A mother reported that her son did not return from soccer practice that day, setting off the search.
"I hope that they are safe. I still have hope. I hope that all 13 of them will be out," said Namhom Boonpiam, mother of 13-year-old, Mongkol Boonpiam who is missing.
The rising water frustrated efforts by the rescuers, including navy SEAL divers, to search farther in the cave, which is thought to be about 6-8 kilometres (4-5 miles) long and contain some large chambers.
By Monday evening, officials made the call to temporarily pull out.
Additional reporting by AAP.