When in battle with your regular bow guns, try to hit your opponent as close to the water line as you can. Spread your missile hits around and he will sink after 2-4 missiles.
If you're having problems with the level "Total War", maybe this can be helpful. In the beginning, you start out with the battleship Carthage and two destroyers.
For both destroyers, turn on your sonar. Turn both destroyers around, and send the Tiger southeast until you reach Monobo Island. Take it out. Take the other destroyer south to destroy the cities of Quezon and Ping Ping.
What I'm saying is use your satellite and since the destroyers can go fast, go try to take out as many enemy cities with them as you can. The battleships are much too slow. If you happen to go in battle between a destroyer and an the enemy's battleships, don't worry. Most times, the enmy doesn't hit you very much. But if you get some damage, retreat and repair it, then blast that ship out of the water!