Their smaller size would be useful in getting through the narrow underwater passage to Pattaya Beach, a rescue team source said.
Hope swelled among the crowd waiting outside the Chiang Rai cave, now cleared of unauthorised vehicles, that the 13 missing people would be found sometime Monday or early Tuesday.
Plans have been finalised for the quick delivery of medical aid to the group when they emerge from the cave.
The front of the cave now has a checkpoint where officials strictly limit attendance to authorised personnel only.
Several unauthorised people – including a woman claiming to be a spirit medium – have been shooed away to keep the area clear for the ongoing operation.