The all-important Scale.

I want to see images like this when my game is done. Only, the whale I have showed you is at the absolute bottom of the scale. I want to see the player getting dwarfed by gigantic sea creatures. A mile-long broadside of blubber and endless rows of cannons gliding past you. Ancient sea monsters rising up from the deep and swallowing your whaleship whole.

All kinds of armed Sea animals, from one-man scouts to mobile animal fortresses. SCALE. Remember how freespace made a big thing about the huge ships compared to your tiny spaceship? Just like that.

This would also open up for multiplayer whaleships, which would be incredibly cool, to just be responsible for one turret on a swimming fortress, or specialized positions like navigation or torpedo stations.