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Blitz will get you more credits and points but won't teach you how to play, and the XP is lower (not entirely a bad thing).

GB2 will get you more XP and horrific credits and points but won't teach you how to play, but the XP is insanely high.

Survive, don't suicide.

For GB2 as DD:

1/ Stay close behind your BB line. The good way is to choose one BB and do her a "little brother". Then protect her against the Subs. Do not OH unless necessary because you will lose your sonarsight then. HH is better anti sub weapon and you can use them also against surface targets but your choice.

2/ If any smaller ship would try to get close to your BB line (ofter work as scouting spy), attack them. (With HH you can even attack heavy cruiser). Your Battleships are often busy fighting enemy BBs and do not have time to protect herself from this kind of scouting. Few seconds can decide the BB vs BB duel.

3/ If battle is going well you can try to choose the right moment (when enemy has only few BBs), go against enemy line and hunt remaining CVs and smaller ships. Your BBs will follow and will clean your path from heavier opponents. Every damage you deal counts and leads to better XP. If you will be lucky you can even sink CV.

There are other possibilities to do, you will find them.
GL

Cool, thanks for the tips.

I didn't realize that flank speed would obstruct sonar, though it makes perfect sense.

For now I'm using depth charges because the HH don't fit into my T slots, and neither do any worthwhile guns. About all my T slots are good for right now are torpedoes or depth charges, so I use torpedoes for Blitzkrieg and depth charges for GB2.

I'll try to be a little more patient and a little more attentive as an escort in GB2, I guess if the enemy subs keep staying back closer to their own BB line, then I'm successful as a deterrent.

Something to keep in mind for DC's: they give you a TK penalty since you get hit by the blast. This is why HH are used to greater effect, plus the fact that they're a gun rather than a torp weapon.

IMHO: grab a ship, gear it up with HH on the R clots and AA on the T slots, and go nuts. You should be able to either deal with subs, or shoot down a plane or two.

Worst case, use your speed & maneuverability to get close to an enemy ship and HH it. Once you learn how to dodge in a DD, it can be a lot of fun to run up to the enemy line, dodging their salvos & nailing them with HH, and then watch your BBs take advantage of the enemy shooting at you and bumrush their line and sink everybody.

I have done sub hunting at low level for 5 nation's grindings.
I prefer depth charge over HHs, cuz DCs give u more power against subs, and with low level gunners, even PHHs have a bad spread which reduce your attack power, not mention the long reloading time of HHs.
At the beginning of a match, check out how many subs the enemy have. Start your ship in the center front position. Do not go full speed (cruise speed), go slow. At the mean time, locate enemy subs position and their heading as fast as you can before the first wave of friendly scouters got shot down. As the game goes on, keep looking for enemy subs,find out their headings and determine which sub you want to engage first, based on friendly subs headings, level of enemy subs, and number of enemy subs in one particular heading (North or South). Always go after high level enemy subs as they impose higher risk for your team. Always go after enemy subs who is heading to an area with no friendly subs or low level friendly subs.
At last, stay behind the battle line, wait for enemy subs to reach your battle line. If you rush out, then you die, and you won't help your team at all.

What ussr1222 said is good. If you really want to get good at sub hunting, you need to know your enemy. I know that sounds cheesy but seriously. If you die early, take some time and stay in game and watch your friendly subs. See how they play. See how a sub moves and attacks, and put yourself into their perspective. Then when you start to think like a sub you will be able to better anticipate their actions, and you will know their probable targets.

Like ussr1222 said, always always pay close attention at the very beginning. You dont have to rush to the line as soon as the game starts, so use that time to observe the location of any enemy subs that are still running on the surface. Make a mental note of their direction. If it looks like a friendly sub is going to intersect their path you can bet they will have a confrontation. Watch out, two subs going at it is quite a cat fight, and they wont think twice if your in the way. If your still alive at the end, don't worry your not a leech, escort the flag or a CV if they don't have sonar, or a BB6/BB5.

Good stuff guys, thanks.

So far I've become a lot more patient at GB2s, and I try to at least note how many subs are starting at the south and north sides of the map for each side. As I get more comfortable in the role I'm sure my capacity for awareness will increase.

I'm pretty hesitant to close on a sub once I've spotted it though, since it almost always spells death. Sure, those torpedoes are a bit of a nuisance, but the real danger is that the ships I escort often have hedgehog launchers and as soon as I spot a sub for them they unleash. I've been sunk more by friendly HH fire while trying to depth charge than I have been by enemy subs.

Either stick with the BB and be ASW or AA or play Bltiz and Gunho

Be careful with HHs, they could potentially put you in debt due to their high credit cost.