I cannot believe open water conditions in the Florida Panhandle can possibly be so bad that an instructor would be so limited in the ability to see students. Where I teach students in Colorado lakes, I am limited to a few feet visibility at times; in contrast, I have never seen anything in any ocean anywhere that approaches that.
I wasn't trying to defend the instructor, I'm not aware of the full narrative of what happened in this scenario anyway. I live here and dive the panhandle regularly and I am not surprised that a student could get lost underwater. I don't know how many students the DM had with them, but if it were more than two that makes it even more believable.
I dove this site where the accident happened at the end of May this year, this was before the annual june grass started, which was very bad for PCB this year. The day I dove it was "decent" conditions and in a few kicks your buddy could disappear. I've found it curious how folks in this area measure visibility too. Someone will say, "oh yeah looks pretty good, 25 ft" and then I get in and almost have to stay arms length with my buddy or they disappear. My rule of thumb is divide by two what most in this area report as visibility.
BTW, this isn't the "ocean". It's a jetty dive, it's an inlet from a bay to the Gulf. The Gulf has a lot of chlorophyll in comparison to the Pacific, Caribbean or Atlantic. And if you dive on low tide... your diving a bay basically, not an ocean.
Just one other thing this is directly from a local dive operations website:
Please note that the Gulf of Mexico does not usually offer visibility necessary to be able to guide a large group of divers as in the Caribbean. If you have never dived in the Gulf of Mexico, to a depth of 90 feet, in somewhat limited visibility, or have not made a dive within the last year, we recommend hiring a Divemaster to ensure that you have a safe and enjoyable experience.
Again, not defending anyone here, just pointing out that it is believable to lose a student diving here, definitely have to be on your A game DM'ing here.