Generally, Petri said, water polo referees call only what they can see. Fouls for grabbing or holding an opponent that can be seen above the water line are the ones that are typically penalized. What goes on underneath the water is virtually impossible to see from the official’s spot on the pool deck, and trying to interpret what is going on based on how players move or react is rarely attempted.

“If they did that all the time, you’d have people just acting or simulating to draw whistles,” said Brenda Villa, the star of the United States team, which has a rematch with Spain in the gold medal match Thursday.

Asked for her most memorable moment of underwater warfare, Petri said she played about 10 minutes of a game topless at the 2000 Olympics, when an opponent shredded her suit as they grappled for the ball but play continued. Left with little choice, she just kept swimming until the next timeout, when she hopped out of the pool and shimmied into a spare.

Petri laughed as she told the story, and Villa shrugged when the tale was recalled to her, saying essentially that it is just a part of the game. “The person that invents a suit that’s not going to move is going to make a lot of water polo players happy,” she said.