I agree. One shot can sink a tanker. That's a huge amount of money. And the negative press will do the rest. Watch oil prices skyrocket.
Actually tankers are extremely difficult to sink. Their cargo is generally neutral to bouyant. Enviro weenies have ensured that they are mostly double hulled. They have the ability to transfer significant amounts of cargo/ballast quickly. They have good fire suppression systems. Their vast bulk ensures that massive amounts of damage must be done using repeat attacks. (Very unhealthy for the enemies crews to fire that second salvo) Few if any would be sunk or significantly damaged (ie cargo lost, since the vessel can be towed clear if neccesary.) I saw mention of the insurance issue. IIRC the premiums were high but not prohibitively so once we began escorting tankers.