By Assin180
Hidden Objective(s): Steal the Japanese recon plane; On the south island (the one that is closest to you) if you zoom in on the map you will find a lone japanese plane. Sail up to it in your sub while avoiding the aircraft flying overhead and you will capture it. You will see a short cutscene telling you that the plane has been "abanoned" and that you can use it to your advantage. Once it is over you will have completed the hidden objective. NOTE: You need to be on the surface to capture it as the water is too shallow. You cannot use your preiscope will submerged to spot it. You also have to get pretty close to it. If you complete this objective the recon plane will be at your disposal later in the mission (it is equipped with two regular bombs as opposed to depth charges).
Secondary Objective(s): Destroy both destroyer escorts; when you are attacking the group of convoy ships at the start of the level you will see two other ships as their escort. They are destroyers and can be very annoying as they can drop depth charges which do a lot of damage to your sub if you are not careful. Taking them out helps you a lot in the long run, but they are quite maneuverable so it may take multiple hits to sink it. If your torpedoes run low just forget about it and sink the four cargo ships. If you sink both destroyers you will have completed the secondary objective.
Getting a gold rankng on this mission (completing ALL the objectives) does not unlock any bonus units, but you might as well get it =).