When you start the game, your destroyer will be in the middle of the water. You will start seeing submarines entering the screen either from the left or right and at varying depths. You have a total of 6 depth charges that can be on the screen at any one time. This means if you fire all 6, then you have to
wait until one detonates on a submarine or the bottom of the sea before you get them back.
Try to aim your shots to where the submarine will be at. This is, in a sense, ''leading'' the submarine. This is especially important if you are trying to get those submarines at the bottom of the sea.
Always try to keep at least 2 depth charges in reserve. This is for those submarines that are close to the surface that like to throw mines you cannot avoid.
Try to keep moving around in your destroyer. If you sit in one place, the submarines can put mines on either side of your ship and prevent you from escaping and adding a destroyer to their kills.
Don't fire your depth charges too close together on one side. The reason is if a submarine is killed by one of them and the other one is too close, it will also detonate. If you were aiming at another submarine with the second one, then your shot is wasted.
In addition to the mines themselves being dangerous, your ship can also be sunk by the effects of the mines. Don't sit too close when a mine detonates or you will be at the bottom of the sea.
If you ''walk'' your shots you have a better chance of getting a good spread and taking out multiple submarines. Kind of like leaving a diagonal string of depth charges the submarines must pass through. Plus, you don't lose your other depth charges to the detonation of ones around them.
Try to get the 500 required for extra time, it isn't too hard if you play smartly.