The Torpedo is now the exclusive ammunition type for both the solo submarine and duo submarine. While you won’t find it as a drop from any known NPC or crate type, it is available directly for purchase from the Small and Large Fishing Village vendors at the cost of 75 Scrap for 2.
As referenced prior, the torpedo, or direct torpedo as it was formerly called fires outwards in a straight line from the submarine, doing considerable damage to anything that it impacts. Unfortunately, due to a lack of crosshair within the submarine itself, they are very difficult to aim to make a successful shot hard to sink.
The torpedo can be purchased and learned via the research table or learned via the Tier 2 Workbench tech tree system. While impractical for most applications, should you find a base in the water with a door that could be impacted by a surfaced submarine, this ammunition type can be used to potentially raid that door.