According to this story from centralmaine.com the pair was fishing near the northern end of Canal Street in Augusta when they hooked the rusty rifle, which was outfitted with a low-power scope, a collapsible Harris-style bipod, and still had three rounds of ammunition in the magazine, along with a good gout of rust.
"I am not able to tell for sure why it's there, but it is possibly related to some type of burglary, theft or other crime," Lt. Chris Read said in the story. "It's illegal to duck hunt with a rifle, so the 'oops it fell out of the boat' doesn't sound too good. You can't hunt from that area within the compact urban zone with a rifle."
The gun was entered into a database of recovered firearms maintained by the National Criminal Information Center, the story says.
The rifle's age hasn't been determined, but the stock is clearly synthetic and largely intact and a local gun shop owner estimated it was made within the last 15 years. Bizarrely, the same gun expert says that bipods aren't used for hunting, when they are actually a common long-range hunting accessory.