Oh, I forgot something - they were also filled with flotation material - IIRC usually cork - so their flotation value is ridiculous. That eel ain't gonna sink 'em. Like she said - a trap. But the idea goes way back to before subs were around - think Trojan Horse a la waterborne .....
Lemme see if I can't find a good reference -
Well, here is one that DIDN'T win - U 123 got her!
http://www.history.navy.mil/danfs/a/atik.htm