Simple really, what it says on the tin.
I recently found an unescourted modern tanker, and with no eels left and my deck gun not fitted, I had to let him go

Can you use AA flaks to sink ships? In the past using my AA, I have:
- set fire to barrels on tankers
- blown up small coastal patrol boats
- destroyed guns on destroyers and armed merchantmen
- blown up the bridge on a damaged merchant
So I assume they can do damage and take oyt certain bits of the ship, as well as sinking the very small ones. But is it possible to sink a ship? Is plugging away at the waterline a waste of time?