On the 8th mission you will gain the use of your first plane with torpedoes.
After destroying the fighters you will start to engage some ships. These ships take three torpedoes to sink. You can only fire one at a time until it reloads. You need to be going slow and flying quite low otherwise the torpedoes sink into the water. When both lights are green on your crosshair that means you can fire.
After firing your first come around for a second and third run after the ship is sunk the achievement will unlock.