It wasn't a ship-hits-it-and-boom type mine. It was a limpet mine. A limpet mine is simply a bomb sneekaly stuck to the side of a ship with magnets by a human. That human, either a diver or someone in a small boat, could place them wherever convenient. Putting it above the waterline has the added benefit of allowing the oil to pour out and catch fire. Oil is lighter than water. So if a hole was made below the waterline then, depending on how full the tanker is, you may have more water flowing in than oil flowing oil.
>Niethr vessel was carrying oil as cargo.
The attackers probably didn't know this. They probably assumed it was crude oil. I would.