A beaver Attacked in the water by a Pike or Musky?? LOL is there Springfield Nuclear plant there?
Can't see a mature beaver attacked by any musky or pike. Muskrats sometimes yes, Baby beavers sometimes yes, Adults highly unlikely.
As Atilla said about winter kill. Sounds like it may be a possibility that it drowned under the ice and then scavenged.
Or it could be a victim of and road mishap. late winter early spring beaver and muskrat bodies can be readily seen along sides of roads. And hwy 35 takes alot creatures lives each year
As for Predators... Wolves are a major predator on beavers. Actually they probably are the main predator. They like to sit near dams and ambush them when they come ashore. And wolves are through out canadian Shield cottage country areas but are very discrete when moving around human development areas. i.e cottages.
With just a hide, some bones and no blood it sounds like that beaver has been dead for a while. and stripped clean by everthing and anything that like meat and or needs calcium. including squirrels, mice, blue jays, crows ,ravens...etc