Although I own SH4 for years, I didn't play it very much, still figuring out what mods to play, currently I give it a go with the patched 1.5 (U-Boots inclusive).
I had good AA tactics in SH3 and SH1, depending if I spotted the plane by radar or if it was already in sight.
If the plane is in sight, it's a very bad idea IMO to crash-dive, this may get you killed, you won't make it safe under water because it takes too long and you lose any control, so you have to fight it out, go flank and present your broadside to the plane until the first bombs hit the water. Just then it's the perfect moment to crash-dive and turn 60-90 degrees after getting some speed.
If you spot a plane by radar, you have the time to crash-dive and head away from your water trail. Do it, because as said above you are not there to play a chicken game
XenonS