I haven't read the article (I'd like to), but I do have a bit of experience shooting small targets from a small boat in the rolling surf.
I spent a great deal of time with the AKNG in Western Alaska as the XO of A Co, 297th Inf, which consisted of Gambell & Savooga on St Lawrence Island, and CO of C Co, 297th which consisted of the villages of Shishmaref (company Hqs), Wales, Little Diomede, Brevig Mission, and Teller.
These villages (and the NG Units) were made up of Alaskan Eskimo's who made the majority of their subsistence hunting marine animals. I had the pleasure of hunting with them on many occasions.
I was amazed at the abilities of these natives shooting from boats in rather rough water, (considering the the size of the boats and Bering Sea winds.).
I was instructed that its not really a trick, its timing. When shooting seals only the small head is exposed. Wound them they sink and are lost, miss and they disappear. Hit them in the head the die and float long enough to be retrieved. The exposed heads are about half the size of a beer can.
It was about timing. The swells rise and fall, the seals with them, as did the boat, but there is a split second when they reach the top of the swell that they stop. So an instant before the swell reaches the the top aim just above their heads, by the time you shoot the swell pauses, as does the seal and you got him.
With a tad of practice I found it wasn't that difficult of a task.
I would like to read the article to compare the differences and similarities.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071