Saman Gunan, 38, lost consciousness on his way out of the Tham Luang cave complex after delivering supplies and could not be revived.
"His job was to deliver oxygen. He did not have enough on his way back," said an official.
The diver had left the navy but returned to help the rescue operation.
Mr Gunan, said to be an avid runner and cyclist, was part of a massive rescue operation which started almost two weeks ago after the group went into the Tham Luang cave.