Don't forget, your objective isn't just to evade, it's to re-engage. Going deep makes that harder. Also, the deeper you are, the harder it is to observe the attacker (sonar just doesn't get it), & the better (more aggressive) skippers ''badly'' wanted to know, to see the enemy, & use his movements to calculate counter-moves. And the attacker will expect you to go deep; if you don't, you've got a slight edge. (Not to mention it's easier to set up to shoot the SOB!) If you've got a lot of DDs hunting you (which was usual in the Atlantic), OTOH, going deep may be the only effective option.

On depth charges, no, it's the lethal distance, not the depth, being quoted there.