Reapers are Draconian submarines that, if detected, can be hit by any type of weapon while submerged.
Their “Submerged Defense” allows them to take only 25% damage from any weapon type that’s not able to hit submerged subs. This means that any weapon, aside from retargeted missiles, depth charges, and torpedoes, will deal 25% damage to detected and submerged reapers. Retargeted missiles, depth charges, and torpedoes will deal full damage. Also, Reapers do have inherent missile, explosive, and ballistic defense bonuses, so this is important to take into account when factoring in “full damage.”