I don't know that a body would stay in the lake unnoticed that long - unless the lake is isolated and very rarely visited by people or it's in the Arctic.
Once the water warmed up in the spring and summer, the body would begin to decay, bloating up and rising to the surface of the lake and sure to be seen (and smelled) by anyone coming to the lake.
Throughout the body's time in the lake, fish and other life forms in the lake would feast on the body. Fingers, toes, nose, eyes, and tongue would be the first to go followed by other areas of the body.