If I remember correctly, they were testing equipment. Incorrectly serviced gear or a some third party defect that lead to the deaths would be a valid issue and worth investigating. But it would be a civil matter, not a criminal one. "Unsupervised dives" are what most people in that area do. As I recall, both of these men were experienced in divers and were believed to be competent to make this dive. One or both of them had an issue and it got out of hand.
As a family member, I would be concerned that they found one diver on the beach and never thought to send a boat out to look for the second diver right away. He may have been just off shore alive, but hypothermic. That he could have been at the surface and sequentially drowned because the police in a coastal town that was frequented by divers never thought to launch a boat to see if he was alone is pretty disturbing.