Harvest might depend on state. In Florida I believe they must be over 20".
Nope, just checked, all harvest of Goliath Grouper of any size is banned in Florida. Got really overfished for a while and they don't reproduce that great, but I understand they've really come back well and there's quite a lot of catch and release fishing for them, I have an in-law that caught one once.
That the guy who died speared one is an indication he probably wasn't the brightest guy on the block in several ways.