Pol Col Kusol Pankerd, a deputy superintendent with the Koh Phangan police station, said on Thursday that the man may have been dead for up to five days.
Pol Maj Gen Apichart said there was no conclusive evidence of the cause of his death and did not rule out homicide.
The rocks inside the backpack could have been used to add weight for diving, but they could also have been used by a killer to weigh the body down, he added.
Police on Koh Phangan are asking all resorts to review their guest registers to determine if anyone is unaccounted for.