Flores was held overnight at the Leverett Circle Barracks. He will be arraigned Wednesday in Charlestown District Court.
State Police said the men had reportedly been drinking alcohol on a dock near the Massachusetts Avenue bridge before Harrison, while either sleeping or passed out, was intentionally pushed into the water by Flores shortly before 5 p.m.
Harrison was pulled out of the river about 20 minutes later.
Flores was arrested and initially charged with assault and battery causing serious bodily injury.