Benthic2, your opinions, which I won`t even quote, are not getting any better. How about waiting until what happened is established? A coronial inquiry, (depending how USA does things) is certain to be held.
There will be an investigation by FWC and local law enforcement. District Attorney would get involved if possible charges to be made. In the US, the Coroner only offers an opinion as to probable cause of death, but doesn't make charges or do other things beyond forming an opinion of cause of death like they do in the UK and perhaps in Australia.
Coroner's report is typically quick, other than waiting for Toxicology reports. In a case like this one the Coroner would primarily check to see if there was anything that might have contributed to the death, other than the obvious. While that's unlikely, it could include anything from a blow on the head prior to the thruster, a stroke, heart attack, or electrocution, drugs or alcohol in their his system. In some cases it's even homicide vs. accident vs. suicide. Coroner's are doctors.
Law enforcement will question all witnesses and others present including any crew members, any other divers or employees of the dive company and other bystanders. There's been no mention who employed the diver and they will examine whether he was legally able to perform the work. Typically on a boat this size there would be more than one diver present.
The big question will be who turned the thruster on and what did they have to do to do so, was there any lock on the system, what precautions were taken.
This may very well lead to another push for stronger requirements and policing of those requirements to dive and clean a boat. There has been an ongoing battle between OSHA and divers over bottom cleaning and requiring the divers to be licensed commercial divers.
After any legal determinations, any criminal charges, then you have all the civil cases. Someone will be held civilly liable for the death. That could be the boat owner, the marina, the captain, the dive company, or a combination and would include any insurers. While contracts for bottom cleaning came up in another thread for very different reasons, this is a reason boat owners need to have contracts on the work.
This shouldn't be a difficult or time consuming case to determine what happened. Assigning legal responsibility may be a very lengthy process.