The Honolulu Medical Examiner 's office has not yet issued an official cause of death .
Surfers who witnessed the accident speculate that the fin on his board may have severed an artery on his upper thigh .
The Honolulu Medical Examiner 's office has not yet issued an official cause of death , but surfers who witnessed the accident speculate that the fin on his board may have severed an artery on his upper thigh .