[[Or, did the tuna take take the man for a deep dive from which he couldn’t recover?]]
Probably the case- Tuna are extremely powerful swimmers- built for speed and power-
[[Was he tethered to a pole so couldn’t release himself?]]
Sounds like he must have been- had the pole been loose- He coudla just let go- but i suppose the line coulda wrapped around him as he got yanked into the ocean and pulled him under too, or the fishing reel or hooks on the pole coulda snagged his clothing too- lot can go wrong-
He was heard to mutter, "From hell's heart I stab at thee; for hate's sake I spit my last breath at thee. Ye damned tuna."