
Make yourself a specialized Deep Sea Rod (from steel/silk?), drop lure into the depths, wrestle for a bit as a might sea beast struggles for freedom, then lug its medium-large liftable corpse onboard to later butcher as you would any other animal. Lots of possibilities for catches: Marlins, Sea Bass, Groupers, Tuna, maybe even mid-size Sharks.
Seafaring requires a significant resource investment and Deep Sea Fishing would be quite difficult and time consuming so the rewards would have to be well worth it. The meat of large fish would have to be at least as FEP-rich as that of cave anglers, but some of the rarer/larger catches would need to have meat comparable to the better cheeses (when dried) to make all the effort worth it. Also plenty of room for new and unique materials and of course curios. Increased quantities of roe/caviar? Shark Tooth necklace? Swordfish sword as a weapon? The possibilities are surely endless.
Knarrs and even Snekks do have enough liftable storage, and the large fish would bear enough meat, that a single trip would be analogous to mass-production, but it would certainly not be mindlessly easy to fill a boat with such powerful creatures. I believe this Endgame Fishing content fits perfectly into the currently vacant Ocean Content niche.