It’s an Atlantic Tripletail (also known as black fish — they turn black with maturity). He’s caught them while fishing (which is better than while golfing). They can run to three feet and 40 lbs. It’s considered good eats in New Orleans. I’m going to check menus around here.
Further research indicates the young ones swim on their sides to mimic mangrove leaves and to facilitate escape. So the languid side-turning I saw was programmed behavior not death throes. Likely, I was viewed as a threat, and the mimicry started. Photo here.
In US waters, Atlantic Tripletails are found from Massachusetts and Bermuda to Argentina, the eastern Atlantic and Mediterranean Sea, from Madeira Island to the Gulf of Guinea, the eastern Pacific from Costa Rica to Peru, and the western Pacific from Japan to Fiji and Tuvalu. They are rarely found north of Chesapeake Bay. They are found on the Gulf Coast from April to October and then migrate to warmer waters during winter. In the spring, Tripletail concentrate just offshore of two particular spots: Port Canaveral, Florida (March–June) and Jekyll Island, Georgia (April–July).