The theory (science?) of continental drift does not conclusively
explain the shape of the Gulf of Mexico. ---Is it only urban legend
that an asteroid hit here, caused this shape, and left remnants under
the ocean that still disrupts navigational equipment today or has
there
actually been research conducted to this end, and if so, what did that
research come to prove?

Dear mmatw – 
	What a very interesting question.   The title of you question refers
to the Bermuda Triangle and the Gulf Coast.   I will attempt to ask
this as best I can since in all actuality the Bermuda Triangle is
outside the Gulf Coast area.

The Following is an exert from the Smithsonian Magazine:
“In 1978 a young geophysicist named Glen Penfield, who was working
with PEMEX, found himself assigned to fly over the Gulf of Mexico.
Using a magnetometer, he was to measure the magnetic field of rocks on
the Gulf floor--specifically off the coast near Chicxulub Pueblo. Like
the findings of earlier PEMEX geologists, Penfield's were intended to
map out the rock composition beneath the surface and determine the
likelihood of finding oil.
But what Penfield's magnetometer let him see was very odd. More than a
mile below the surface of the Yucatán Peninsula, and for 70 miles out
into the Gulf of Mexico, was a saucer-shaped underground structure
with a magnetic field different from that of any known volcanic
terrain. It also had a most un-volcano-like symmetry. Put together,
the old land data and the new underwater data indicated the existence
of a huge ring, about 120 miles in diameter, half on land, half under
the Gulf of Mexico. It was ten times the size of any volcano, with an
upward bulge at its center similar to those seen on known--though much
smaller--impact craters. “
The article continues on to say that in 1990 it was in fact proven
that this crater was created by the impact of an asteroid.   So no the
impact theory you are referring to is not an urban legend it is in
fact real and true.   Most of the studies done on the impact revolve
around the extinction of the dinosaurs, as there is some correlation
to the time of both events.   The crater that was made by the impact
of the asteroid(s) does have a distinct magnetic field to it.   It was
first discovered by an oil drilling company whom kept the findings as
proprietary information.
Now as for the Bermuda Triangle there are more theories than fact on
how and why it exist or if it even does exist.   I have attached
several links to sites that might be of interest included a map of
where it might exist if you believe its there at all.   I am going to
leave you with the links for now but I am more than willing to expand
on any of them if you desire that info though its not in the question
itself.
Additional resources:

The Object at Hand - A Tale of Two Rocks
http://www.smithsonianmag.si.edu/smithsonian/issues98/apr98/object.html

Continental movement, plate tectonics, continental drift, geophysics,
earth
http://www.aaronc.com/pelgnet/pelchap6/Chap6.html

The Bermuda Triangle
http://www.uiowa.edu/~anthro/webcourse/lost/bermuda-triangle.htm
The Mysterious Bermuda Triangle -  
http://www.cat.cc.md.us/~mpowers/tmbt.html

Search strategy:
Gulf Coast Creation Meteorites
Bermuda Triangle creation theory
Gulf Coast Continental Plates


Hope this all helps and thanks for the interesting question. 

Morningstar

Whoops, didn't realize I hadn't rated this.  Great answer!  Thank you.

Additional interesting links about the Chicxulub Crater:

http://miac.uqac.ca/MIAC/chicxulub.htm
http://www.space.com/scienceastronomy/planetearth/chicxulub_crater_001219-1.html
http://neo.jpl.nasa.gov/images/yucatan.html
http://www.lpl.arizona.edu/SIC/impact_cratering/Chicxulub/Discovering_crater.html
http://web.archive.org/web/20000520094529/http://diamond.ge.ic.ac.uk/jb03/jb03/web/chix.html
http://www.agu.org/sci_soc/sharpton.html
http://www.geophysics.dias.ie/~gkenzie/chix/
http://web.archive.org/web/20011129113848/http://www.star.le.ac.uk/edu/solar/tercrate.html#intro

Morning star is correct, the center of the Burmuda triangle is more
than 500 miles East of the center of the dinosaur killing asteroid
hit. There has been some recent evidence that asteroids often travel
in pairs, so perhaps the smaller of a pair hit in the Bermuda Triangle
the same day.   Neil

Is answers.google posting comments this AM?

The Bermuda Triangle legends have about the same average credability
as ET, alien abductions, big foot, ghosts, channeling, the
Bildabergers and conspiracy theories.

Pseudo-information.  Yes, the Chicxulub impact site has been
correlated with the extinction of the dinosaurs, and yes, it's round,
but NO, it has zero to do with the shape of the Gulf of Mexico, which
is a typical tectonically-squeezed oceanic basin, like the
Mediterranean, Caspian, or Caribbean seas.  The Chicxulub site is
about 120 miles in diameter (and is a structure buried under more than
5,000 feet of more recent rock layers, which don't show that structure
at all), and the Gulf is approximately 1000 miles in diameter.  The
impact site is a tiny blip on the edge of the Yucatan Peninsula
(here's a map showing the main crater ring
http://neo.jpl.nasa.gov/images/yucatan1.jpg), and isn't even visible
at the surface.  Regarding "impact remnants" which are supposed to be
"disrupting navigation" Chicxulub is a typical impact in which the
impactor (meteorite) is vaporized on impact.  The dust and vapor was
scattered across much of the world, and there are no significant
remains at the impact site.  Finally, as far as it having anything at
all to do with the Bermuda Triangle (which the previous commentator is
right in classing with alien abduction legends), it's over 2,000 miles
away.