Jon Bon Jovi has finished shooting Universal's submarine epic "U-571" -- and are his castmates and director ever impressed!

Turns out Bon Jovi is claustrophobic, which means he was courageous -- or crazy -- to tackle playing a World War II submariner in the first place.

"At one point, he had to get into a small container, a piece of a set about 4 feet tall, with four other guys," reports filmmaker Jonathan Mostow. "There was a problem with an electrical short, and a little fire broke out. ... I would say the bravest thing Jon did was going back into that space for the second take."

Turns out there was lots of bravery on the production, which also stars Matthew McConaughey, Bill Paxton, Harvey Keitel and David Keith.

"There were dangerous, physically demanding shots. Nobody complained, nobody chickened out. ... There was a certain healthy competitiveness among the cast," says Mostow, who also wrote the thriller. "No one would say no to anything I asked of them. The conditions in Malta were horrible -- cold weather, cold water pelting down on them, working on the water. After the first night, I was afraid the cast might mutiny. Instead, they acted as if it were the most fun they'd ever had. We did a month of six-night weeks, on the water, with massive explosions, gunfire and artillery fire."

The cast also endured major battering in a simulated depth charge attack for which their set was mounted on a $1 million gigantic hydraulically controlled gimble and spun around.

With months of intensive post-production work on "U-571," there's no telling yet when Mostow's ship will come in -- to theaters.