So I have some idea about the heroine, now about the hero. Shark man for lack of a name at the moment.
Shark man spends most of this time as a never-seen-before species of shark (I’ll have to come up with that). This shark is like a Great White and a Bull Shark. Aggressive, strong jaws, super predator. Doesn’t necessarily like to eat man, but won’t back down from a fight either. Seals taste better.
Shark man is intelligent though – he has a strong sense of family and possession. What’s his is his, what’s yours might be his if he needs it. He’ll fight to protect his civilization. He’ll fight just because if he’s provoked. But he’s sneaky. He knows how to plan an ambush – which is what brings him to his human form. He’s planning an ambush of the crew that is getting too close to discovering his cities. He’s not the only shark man out there, but he’s the shark man on the spot. Maybe he’s some sort of guardian in his society.
His people worship the shark god and volcanoes.
There are three main areas (hidden cities) where his people gather – off the coast of Western Australia near Perth, off the western coast of South Africa (Mossel Bay), and near Carmel, CA. The cities are deep in the ocean but the entrances are closer to where treasure hunters or divers look for shipwrecks.

Using the 12 archetypes info again, shark man is a Caregiver (to protect others), a Hero (prove his worth through courageous acts), and The Sage (to find the truth). I’m not set on these three – his goal is to protect his secret. He’s aggressive, powerful, crafty, determined, and of the ‘kill or be killed’ school of thought. Not very trusting, this guy. One of the conflicts I envision is his reaction to the Marine Biologist’s profession. He knows she dissects things and doesn’t want to be one of her lab tests. He doesn’t believe that she’d never do that to him – so she has to convince him that he can trust her. The captain bad guy, on the other hand, would happily sell shark man to the nearest lab after he gets credit for capturing him.