http://breakingnews.iol.ie/entertainment/story.asp?j=161779900&p=y6y78x6x6

Harry Potter star Daniel Radcliffe took an intensive six-month scuba diving
course while preparing for new movie Harry Potter And The Goblet of Fire, so
he could hold his breath during an underwater scene.

Boy wizard Potter becomes a fish in the new film and Radcliffe spent weeks
perfecting the scene.

He says: "I had the most amazing stunt team who I trained with for six
months and who were down there in the tank with me.

"I was sharing somebody else's air from their scuba diving tank and we both
had regulators. They'd say 'Three, two, one,' and on the 'three' I'd blow
all the air out of my lungs and on the 'one' I'd take in a very big gulp of
air and then it's all about how much action you can do with that amount of
breath in your body.

"The hard thing was not just holding my breath, but I wasn't actually
allowed to let any of the air out because Harry is supposed to become a fish
with gills; there's not supposed to be bubbles going around. So if I look at
all pained, you know why."