Mutated Bowhead Whale
The final boss. Named identically across every source checked, and the fight itself is independently confirmed by an official Steam achievement.
How to Find It
Reported to require a defeated (non-mutated) Bowhead Whale as bait, a single-source detail (Destructoid), kept community-reported-level confidence even though the boss itself is verified.
How the Fight Works
Fight it normally with guns until its health is nearly gone, then switch to your fists to land the final blow. This matches the official Steam achievement "Handyman: Defeat the final boss using only your fists."
Rewards
Completing this fight is described as the last step before reaching the mainland.
Not yet spotted by name in the two official trailers inspected this round. Verification rests on three independent guide sites plus the achievement text, which already meets this project's bar.
Related
Boss Fights & Island Progression
Islands unlock sequentially: defeat the boss guarding your current island to progress to the next, more dangerous one.
Volcano
The final island area, home to Goblin Shark, the Bowhead Whale, and the Mutated Bowhead Whale final boss.
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.