In my opinion, the destroyers are smarter in the later years, compared to SHIII. Especially their search patterns once you are spottet
I was spottet trying to take out a few huge tankers that was a part of a larger convoy, took a few of them out, and they hunted me down even though I was on silent running and had deployed decoys. One of the three pinged me badly, and the other two then joined up and rest is history ...
A little sidenote: It doesn't exactly make it easier that most of the chinese see is insanely shallow water, and hard to hide.
Great game by the way, even though it lacks a few features from SHIII.
And I agree with 9th Cow on the convoy part, there are so many that you don't have to search long for them.