Blue marlin

Catch Tuna and Marlins off of docks. For the coelacanth catch when raining. All have large shadows. You can bait but I just look for large shadows.

For the Marlin, just toss some fish bait right beside the pier on one side of your island, and just wait for a large shadow as Dietrich said.

Yep pier. That is what I meant by dock. Hope I did not confuse anyone.