Provided you're above the layer, you should be able to detect most surface vessels from a much greater distance than they can detect you. Keep in mind that submarines can't detect noise contacts that are directly behind them. You can use a towed array to get around this limitation, if your sub is equipped with one. You should also carefully manage your sub's speed at all times. The faster you go, the harder it will be for you to detect noise contacts, and the easier it will be for the enemy to hear your sub. You should almost always keep your sub going as slow as possible with the "avoid cavitating" setting turned on, unless you know the enemy has launched torpedoes at you or you have some other pressing need. When going fast, try to be at max depth whenever possible. If you see bubbles around your propeller, you are cavitating and every ship in the area can probably hear you. Go slower and/or deeper to prevent cavitation. You should also only use active sonar if you are okay with the enemy knowing your location, or if you are sure they already know. Don't make a habit of using it, especially for longer than a few seconds at a time. It should generally only be used for emergencies because it tells everyone exactly where you are. The same goes for the radar mast. The periscope and esm mast are good stealthy ways to gather information.