Plum police say 51-year-old Gregory Rudcki of Monroeville and his companion left the Springdale boat dock at about 8 a.m. Saturday.
Lt. Larry Conley says the pair were fishing but had to make repairs to their fishing tackle, so they decided to cross the river to the Plum side where the dog could swim in shallow water.
Conley says Rudcki entered the water but began to struggle staying afloat. The other man threw him a flotation device and Rudcki started swimming toward shore, but then went under and didn't resurface.
The victim's body was recovered at about 2 p.m. Saturday. County police and the Pennsylvania Fish and Boat Commission are investigating.
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