Most "areas" are awfully bland so I didn't vote. Hopefully XY will sort that out. The sea could have coral reefs (assuming real-world animals exist in the Pokémon world), the caves could have various geological structures and maybe cave art, etc.
Hmmmm......the sea is usually annoying but that can easily be taken care of with a few repels. Caves on the other hand is where you more than likely need plenty of HMs like Flash (I know it's not an HM anymore but it's still required to pass certain areas) and Rock Smash, which is annoying.