Yes this is very very very cool. Makes me think of Jules verne, the James bond film where they had scuba divers fighting, and WW2 Italian and British chariot submarine raids
Are you intending to create a tech tree based on underwater concept? If so, and if my creative ideas fit in with the theme then I would gladly contribute a faction.
if you do create a new underwater pack, I suggest you make all infantry/people the same size as annex, about half the size of megaglest. I think that scale looks and feels much better