Board Thread:Questions and Answers/@comment-30620272-20161127223719/@comment-99.74.214.79-20161128033922

I assume your ships aren't equipped with Anti-Submarine weapons. If they aren't, they will do little to no damage to submarines.

You should come back when you have a decent amount of depth charges and a few sonar. Sonar are a bit harder to craft, but depth  charges are pretty easy to come across if you set a Destroyer or Light Cruiser as your flagship and use the recipe 10/30/10/10.