I'm sure this is not a new idea.
Rare can honestly copy and repast the megalodon to add this new boss. Reskin the whole character to look like a whale. The Meg already swims close and around the boat. Instead of the charging with open teeth effect from the meg, the whale would simply slap the boat while swimming alongside the ship.
The whale could have a shadow of birds overhead so we could track it. Every now and then there could be a spout of water as a cool effect. The whale would have to be killed only by spears. Yes, they should add spears. A one and done throw. Once the whale is killed, barrels, whale fat (cookable), ship parts, and other items could float up just like defeating the meg.
Ide honestly be satisfied with this new boss, a simple reskin of old code already in the game. Let me know what you Pirates think?! CHEERS!!