To Lord Dragos - Greetings!
Think of the combat in this game in two forms - knife fight inside a telephone booth and ranged combat.
All knife fights are resolved to the death - one side or the other will win. With ranged combat, you have can withdraw after shooting. Call it shoot and scoot. Cruisers have the shortest range so you have to be careful with them. The enemy closed in, your cruiser fired at range and then he closed into knife fighting range and picture 1600's pirates pulling up alongside your ship and boarding it...
How to kill subs - air patrols with fighters with bombers waiting at closeby aerodromes ready to launch once a sub has been found. Otherwise 20 inf at sea can engage one sub at 1-1 odds.
Sub has a strength at sea of 4 = almost a battleship or 2 cruisers.
2 subs can sink a battleship. So attach some cruisers with battleships to act as escorts and provide additional anti-aircraft and anti-surface/sub defense.
Hope this helps.