@Sprouto Alright, fair enough.
If I were designing the game from the ground up, this is how I would handle submarines:
-Submarines (when submerged) are only vulnerable to torpedoes and depth charges
-There's no such thing as "underwater vision". You can see your own/allies' subs, but submerged enemies will never be visible. If you have sonar coverage then you'll see a grey "submerged unit" icon. The only exception would be EXP units, they'd have the circle icon. Sonar can see how big it is, but that's it. No indication of which enemy player or its remaining health. No way of telling if it's even a sub or an assault bot walking across the sea floor.
-Subs don't provide vision either, only sonar. So they can only tell the location of surface ships, not whether it's a cruiser or destroyer or which player owns it.
-Make destroyers/torpedo boats and UEF Battlecruiser more anti-sub specialized. How this is done would vary between factions. They could have better torp defense, use depth charges instead of torps (meaning a sub's torp defense does nothing), built-in sonar... I'm sure there are other possibilities.