Aiming below waterline...viable tactic?

[Hmuda]

I noticed that when I use the deck gun and aim lower than the waterline of the enemy ship, I still see the flash and hear the explosion of the resulting hit even though the projectile has to travel the last few yards under water.

So, should I always make sure I hit them right on the waterline or can I go for the "below waterline" approach as well? The shells seem to puncture the hull just as fine, just want to know which approach is the more...ammo efficient method.