Steve, I don't recall ever seeing any indication that US fleet boats could (or did) receive radio messages when as deep as 80 feet. And I believe u-boats did so, routinely. I suspect this may have been a result of the fact that the fleet boats operated at much greater distances from base. Pushing a radio signal through water takes a lot of power. Power falls off with the square of the distance from the transmitter. Double the distance and you get only one-quarter of the power. So getting a message to a sub 4000 miles from home would take four times as much power as would be needed to send the same message to a sub 2000 miles away. Another way of looking at it is that the farther sub would have to be a lot closer to the surface to receive the message. Which seems to match the first-person reports of American subs usually getting messages when at or near periscope depth and the German subs receiving messages when much deeper.
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