I'm trying to find the name of a movie that I saw when I was a kid,
back in the late 70s - early 80s.  It involved a boy who would free
dive, without a scuba tank, to collect pearls from oysters.  They
might have been black pearls.  I also remember there being stingrays,
maybe a giant stingray, that was either threatening, or friends with
the boy.  I've already checked the matches for "Black Pearl" in
www.imdb.com, but nothing seemed to fit the description.

Hello.

I'm going to go out on a limb here and say that your movie actually
was "The Black Pearl" (1978) directed by Saul Swimmer.  The movie was
based on a book by Scott O'Dell.

The New York Times' summary matches your description:

"The hunt for a large, precious black pearl located off the coast of
Baja California provides the basis of this youthful adventure. Though
a relatively simple dive is required, the young hero must first
contend with the giant, protective manta ray that guards the
treasure."
http://movies2.nytimes.com/gst/movies/movie.html?v_id=5915

"Black Pearl
Directed By Saul Swimmer
Starring: Mario Custodio, Gilbert Roland, Carlos Estrada
In the enchanting Mexican village of La Paz, young Ramon Salazar
strives to achieve manhood by becoming a pearl diver. During his
training under Soto Luzon, an old Indian of great skill, Ramon
discovers more about life than he ever could have imagined."
source: tromamovies.com, cached by Google
http://66.102.7.104/search?q=cache:o9CS1knPZ4UJ:international.tromamovies.com/movies/blackpearl.htm+%22black+pearl+%22+&hl=en

"When young Ramon finds a giant black pearl beneath the seas of Baja
California, he angers the giant manta ray who guards it. Neither Ramon
nor his father understand the danger that such a pearl can bring. Both
the Manta Diablo and the Sevillano (another pearl diver) are
determined to get their hands on the pearl, even if they must kill
Ramon to do so.
 The Black Pearl was released as a major motion picture in 1978. The
film starred Gilbert Roland, Carl Anderson, and Mario Custodio. A
Royal Production Corporation picture. A William Cash/Saul Swimmer
Production. Directed by Saul Swimmer."
source: Author Scott O'Dell's official site:
http://www.scottodell.com/bks/perl.html

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search strategy:
imdb.com, "black pearl"
"black pearl" "saul swimmer"
"black pearl" "manta ray" "carl anderson"

I hope this is the movie that you had in mind. If not, please let me
know via the "request clarification" feature.

Thanks for anwering the question so fast!  This is definately the
movie I was thinking of.  I even remember there being something about
the ray being named "Diablo," but failed to include that information
when I asked the question.  I guess I didn't recognize it on IMDB
because they don't provide a description of the movie.

Thank you for the tip.
-juggler

he the movie name is 
Silent One 
guy in the movies is dumb