Corroborated
Volcano
The final island area, home to Goblin Shark, the Bowhead Whale, and the Mutated Bowhead Whale final boss.
"Volcano" is corroborated by two independent fish-list sources. Reported as the last stage before reaching the mainland. Five endgame fish (Anglerfish, Blobfish, Oarfish, Superdwarf Fish, Stonefish) are reported here, caught with the Scientific Lure.
Related
Mutated Bowhead Whale
The final boss. Named identically across every source checked, and the fight itself is independently confirmed by an official Steam achievement.
Goblin Shark
A reported Island 5 boss, summoned with a Scientific Boss Lure.
Bowhead Whale
A reported Island 5 boss, distinct from the (verified) Mutated Bowhead Whale. Defeating it reportedly produces the bait needed for the final boss.
Scientific Lure
Island 5's bait tier, for the endgame fish (values reported in the 1,360-1,700 range).