[GhostSoldier]

Heavy, steel, iron, etc., etc. objects can reach speeds over 400 mph if they descend in sufficiently deep water

400 MPH. You sure? That sounds quite a bit faster than the free fall rate through the air (terminal velocity...I think is in the neighborhood of 286 ft per second).

[muawiyah]

Only counts for low density stuff. Now you get a really dense nickel-iron meteor, whizzing along at 15,000 mph relative to Earth, and it is NOT GOING TO SLOW DOWN to 286 feet per second as it enters the atmosphere and punches a hole in a car in Australia or New Zealand!

[Swordmaker]

Terminal velocity in air and water depend on cross sectional density more than anything. In air, the terminal velocity of a feather is considerably different than the terminal velocity of an anvil.

The "antenna" could be nothing more than something dropped overboard that impacted vertically and is stuck in the muck.

Where is Robert Ballard when you really need him???

[Hermes37]

A human in flat stable freefall reaches around 144 feet per second.
This is about 120MPH
200+ MPH is possible if U go into a head down dive.
A penny in freefall will go thru a car if it hits it.
A bullet in freefall will approach the same speed going down that it had on the way up. (just as deadly)

A heavy metal object with a small cross section relative to the direction of travel could easily fall through water fast enough to stick upright.

Due to it's shape, it would be nearly impossible for it NOT to be straight upright (or exactly upside-down).

[Rudder]

That sounds quite a bit faster than the free fall rate through the air (terminal velocity

Yes it does and it is faster in water. Check out Bentley's Thresher Disaster. There are abundant data which show that some descent rates exceed 400-600 mph.

Simply amazing, ain't it?