What other things could surface ships do to attack a submarine?

One option was to force it to the surface. However, as long as it was not badly damaged, the submarine would rarely surface willingly. In those cases, it may have been possible to force a boat to the surface by tracking it until the battery runs out or the air was no longer breathable. Then the submarine would have to come to the surface or suffocate.

Once the boat was on the surface, the ASW vessels could ram the submarine or attack with guns. They might have even been able to fire a torpedo at it.