Although athletic stand-ins and special effects trickery are commonly
used in films, that was apparently not the case here. According to a
reputable source, Warner Bros. Online, Kevin Costner did his own
golfing in "Tin Cup." The repeated shots into the water were achieved
by editing. It is not uncommon for certain scenes in movies to be
re-shot dozens or even hundreds of times, with only the best shots
making it into the final cut. Several days of shooting may result in a
few seconds of on-screen time.
"But the presence of all these top pros also presented a problem:
Could Costner's swing look strong enough to believeably exist in the
same league with these guys? The filmmakers assigned McCord and Kostis
to work with Costner and, to a lesser degree, Johnson (who was already
an eight handicapper and a regular on the celebrity golf circuit).
Shelton had thought all along that he would have to use a double for
most of Costner's swings but the athletic actor proved to be a quick
learner. The result was that Costner did all his own golfing in the
Warner Bros. Online: Tin Cup Production Notes
"Tin Cup is such a grown-up, witty, pacific, all-American pleasure
that it's damn near impossible to resist, even for people to whom
watching golf is tantamount to watching a glacier move. (Fans, on the
other hand, will note that Costner does all his own swinging.)"
Movieline: Tin Cup
"Director Ron Shelton had thought he would have to use a double for
most of Kevin Costner's swings, but the athletic actor proved to be a
quick learner and did all his own golfing in the movie."
TNT On Location: Tin Cup
Here's an interesting bit of trivia:
"The scene at the end of the movie where Kevin Costner hits the shot
into the water hazard again and again was based on an actual event.
Gary McCord the commentator with the handlebar mustache in the movie
is an actual commentator and former pro. In a tournament he had a
similar shot to Costner's. He needed a birdie to win and went for it.
He shot over and over again and finally got it in 15 strokes. In the
movie Costner gets it in 12."
Internet Movie Database: Tin Cup
My Google search strategy:
Google Web Search: "tin cup" costner "his own golfing OR swinging"