the mk 1 eyeball isn't going to find an aluminium tube 4 miles underwater
If the aluminium tube remained intact hitting the water at terminal velocity!
Even in a controlled ditching, the tube tends to get holes in it and floaty bits fall out or sticky out bits fall off and then, float.
^^^^^^^^^^^^^^
Aluminium tube was found on the bottom, even my mk1 eyeball with corrective lenses might spot that from the beam/observers window.