Megalodon

Wild

Were it not restricted to the waters, Carcharodon ultramegalodon would be one of the most dangerous creatures on the island. As powerful and dangerous as the Tyrannosaurus is on land, Megalodon is near its equal in the water. In addition, it has a speed advantage over any non-aquatic creature when submerged. Megalodons need large quantities of food to sustain themselves, so they attack most creatures immediately on sight. Smaller fish are the sole exception I've seen; I believe this is because they cost more energy for Megalodons to catch than the predator would gain.

Domesticated

Having access to the resources and treasures hidden deep within the ocean is near impossible without a domesticated sea creature. The Megalodon, though difficult to domesticate, proves to be very useful when exploring the deep sea. It's not the most efficient swimmer but it should be able to protect your cargo should you find yourself in a hostile encounter.