I live in a city (Manitowoc, Wisconsin) where we are fortunate to have a Gato-class submarine (The USS Cobia) moored in the river mouth downtown. While it wasn't built here, 28 others of its class were. As we passed by it the other evening, my 9-y-o daughter remembered how I'd told her it sank a few years ago in the river. The Manitowoc River being about a foot deeper than the draft of the ship, almost no one noticed ... Then she asked how deep it could go. I wasn't sure, but the library is across the river and we were in the parking lot, so we went inside and quickly found the information. The maximum dive depth rating was 300 feet.
Waitaminnit. The ship is *312 feet long*. It suddenly dawned on me that this thing, if stood on end, was beneath its maximum dive depth rating! I walked back outside and looked across the river at it, trying to grasp the scale of what had actually been going on during the war. Wow. Needless to say, most of those movie depictions we've seen for decades are basically wrong - or at least out of scale.300 feet is pretty damn 'deep' water by Great Lakes standards (only Superior has a deeper average at 500 feet, while 300 is deeper than the maximum depths in Erie and Ontario). OTOH, Wikipedia references put the average depth of the Atlantic at ~12,000 feet and the Pacific at ~14,000. So all those subs were 'skimming' around at less than 300. They certainly weren't lurking near the bottom (except in continental shelf areas averaging ~450 feet) as some films might have you believe.
A little Googling around finds U-boats at about half this length (if you're more familiar with the U-505 in Chicago as opposed to The Cobia) with maximum depth ratings at about 450 feet, or three times their length.
I don't know about you, but I'm still trying to wrap my brain around this.
-- C.R. Krieger (Been there; done that)