Believe U Me, if the real u-boaters could go to 300m, more than 25% of them would have come back, but they didn't! U-boats can't just dive to 300m to get below ASDIC, its just not possible, and I believe the game reflects this accurately.
Again, the casualty statistics show, If they could they would have, but they didn't because they can't.
Nice info though about the Whales, water and all that, I would love for a whale to just swim into the ping of a destroyer and save my skin!