On the topic of this thread i have thrown together a mod that gives navy a strategic missile defense ability, it's simple but effective.
but if your interested its called " Naval Anti Nuke"
I have basically created a clone of the Missile subs and sera battleship but removed the nuke attack and given nuke defense, all other aspects of the units remain the same.
The nuke defense subs have shorter range from static defense from 90 down to 70 and the seraphim battleship is 60
and they are surface fire only!
the submarine storage is 3 and the seraphim battleship is 1
the battleship is different as people make a whole load more battleships than missile subs.
I made the mod quite quickly so i am aware of somethings that need improvement but if you use the mod please feel free to leave me any feedback.