[Zen]

Because most units who have Fire magic don't have water magic that enables them to go underwater. Rod's of the Phoenix are always a threat, but since you are underwater you have to drag a fairly meaty army or protection force with them if you don't want your Commanders swarmed by tritons.

Whereas Shock Resistance is always good and the majority of air spells work underwater. Also certain shock spells cause fatigue damage which is a big no-no for Sc's. It's purely a preference thing, thats why you probably want a Hellsword/Fire Helmet combo if you can get it to cover all your resistances.

If you happen to be contesting Abysia/Marignon/Machaka underwater you might want to look to fire resistance a little more.