Christian Yelich gave boaters in Pittsburgh's Allegheny River a reminder Friday: If you sail too close to the Pirates' ballpark, prepare to be boarded.

While practicing for Monday's Home Run Derby (8 p.m. ET, ESPN), the Milwaukee Brewers slugger sent a bombardment of balls out of PNC Park and toward those enjoying the holiday weekend on the river.

With an MLB-leading 31 homers and counting, it hardly seems Yelich needs to practice for the event, but he took some soft tosses from former Marlins coach Pat Shine around 2 p.m. ET on Friday, with the aim of hitting them out of the park.

One of Yelich's massive shots broke a window of a houseboat, according to a report from Fox Sports Wisconsin, to which Yelich responded with a shrug emoji.

??‍♂️ https://t.co/si2mm6EOpY

— Christian Yelich (@ChristianYelich) July 5, 2019

Boat owner Clint Lohler estimated that at least four balls landed in the water near him, and he fished out one as a souvenir.

"When it started out, I thought it was a young kid throwing the ball in the river," Lohler told the Pittsburgh Post-Gazette. "I was like, 'Oh, there's a baseball.' So I went and got my pole. ... The next thing I knew, they were coming over the wall here and hitting the boats."

One concerned boater called police, according to multiple local reports. Allegheny County District Attorney Steve Zappala joked on social media that he would love to bring Yelich in for questioning regarding the incident and suggested 7:05 p.m. ET -- the start time of the Brewers' game against the Pirates at PNC.

"The legend of Yeli," Yelich's teammate Ryan Braun said before the game. "That's how legends are made."

PNC Park is on the Allegheny River in the North Shore neighborhood. Boats dock on the river wall, and commuters to the neighborhood often park their cars elsewhere and travel via boat to work or to attend games.

The boaters had cause to be surprised. Since PNC Park opened in 2001, only five home run balls have landed in the river during games. Two of those were hit this season by the Pirates' Josh Bell -- who will also be among the eight contestants competing for the Home Run Derby's $1 million price next week at Progressive Field in Cleveland.

"You have to understand there's some inherent risk," Braun said. "Especially if you have guys like Yeli and Josh Bell practicing for Home Run Derby. I think there's probably added risk if you're parked anywhere on the river with your yacht or boat in that situation."