I think Jim is right there. The homicide detective is simply the most qualified on the local force to investigate even if there is no suspicion of foul play.
So the Looe Key accident involved two divers alone a private boat? It does not sound like the lack of a separate pilot was part of the problem from what I have read, and it sounds like a medical issue maybe...?
I dived from the Minnow once on my first trip to the Keys and was impressed enough that I used that Op for newbies when I organized a two part group later - the rest of us on other boats for deeper wrecks. I don't remember his name so don't know if it's still the same skipper?
So the Key Largo death sounds like it was caused by the MOW? Aka, bluebonnet, is not a true jellyfish but a siphonophore, which differ from jellyfish in that they are not actually a single creature, but a colonial organism made up of many minute individuals called zooids. Wiki further reports them responsible for 10,000 human stings in Australia each summer, severely stinging, allergic reactions possible, deaths rare but possible - and with a lot of pain plus a feeling of entrapment, perhaps panic and downing are risks?