Submarine can only be aimed by destroyers and submarines.

core/src/com/unciv/logic/automation/UnitAutomation.kt

@@ -118,6 +118,12 @@ class UnitAutomation{
         if(tileCombatant==null) return false
         if(tileCombatant.getCivilization()==unit.civInfo ) return false
         if(!unit.civInfo.isAtWarWith(tileCombatant.getCivilization())) return false
+
+        //only submarine and destroyer can attack submarine
+        if (tileCombatant.isInvisible()
+                && (!unit.hasUnique("Can attack submarines") || unit.civInfo.viewableInvisibleUnitsTiles.map { it.position }.contains(tile.position))){
+            return false
+        }
         return true
     }
 

core/src/com/unciv/logic/battle/CityCombatant.kt

@@ -12,6 +12,7 @@ class CityCombatant(val city: CityInfo) : ICombatant {
     override fun getTile(): TileInfo = city.getCenterTile()
     override fun getName(): String = city.name
     override fun isDefeated(): Boolean = city.health==1
+    override fun isInvisible(): Boolean = false
 
     override fun takeDamage(damage: Int) {
         city.health -= damage

core/src/com/unciv/logic/battle/ICombatant.kt

@@ -1,6 +1,7 @@
 package com.unciv.logic.battle
 
 import com.unciv.logic.civilization.CivilizationInfo
+import com.unciv.logic.map.MapUnit
 import com.unciv.logic.map.TileInfo
 import com.unciv.models.gamebasics.unit.UnitType
 
@@ -14,6 +15,7 @@ interface ICombatant{
     fun isDefeated():Boolean
     fun getCivilization(): CivilizationInfo
     fun getTile(): TileInfo
+    fun isInvisible(): Boolean
 
     fun isMelee(): Boolean {
         return this.getUnitType().isMelee()

core/src/com/unciv/logic/battle/MapUnitCombatant.kt

@@ -11,6 +11,7 @@ class MapUnitCombatant(val unit: MapUnit) : ICombatant {
     override fun getTile(): TileInfo = unit.getTile()
     override fun getName(): String = unit.name
     override fun isDefeated(): Boolean = unit.health <= 0
+    override fun isInvisible(): Boolean = unit.isInvisible()
 
     override fun takeDamage(damage: Int) {
         unit.health -= damage