This is where tweaking things in the career files might help. Lower the renown a person gets for sinking things. Especially airplanes. Why should you get renown for downing an airplane, when technically, a submarine should never be in a duel with a plane, and in practice, is not to be seen by one, much less shot at by one? So you bump up the renown awarded from completing objectives, and lower the renown received for other things. A ship sunk no longer gets you 200 points of renown, but 20 - unless it's a carrier or the Yamato, of course...
- I haven't tried it yet, but can you give a player negative points for failure to complete an objective? What method would you use for failure? I've tried that, and it's entirely too easy to have a "fail" if the player does things "out of order".