Decks awash is about 30 feet in stock. Try a couple of feet up or down from there. You're looking for the deepest you can run but still be on diesels. I'm a strong believer in having 100% charged batteries at all times. Your top speed at decks awash is 10 knots. You can submerge almost instantly. All the crew hocus pocus is completely unnecessary and has no effect on submarine submergence behavior.
I love to shoot from decks awash. It makes you less visible, makes you a smaller target, reduces dive time to almost nothing and gives you all visual, periscope and radar abilities. All you give up is 10 knots of speed. You're close to the enemy so you don't need that.
