Here’s an idea.
A random boss encounter while at sea, a giant white whale like Moby D.I.C.K. you can either fight and kill it or escape it but it will attack again sooner or later.
Edit: It could even be a mutli-encounter battle, once you do x-amount of damage to it it would retreat and then randomly it’ll attack you again with the same amount of health remaining as the last encounter and this will continue until the beast is dead.
You get its head as a trophy for your base and you earn the “For Ahab” Trophy/Achievemt.