Building A Mystery

When kayaking on Saturday in Glasgo Pond in CT (freshwater) I saw these things attached to a tree branch underwater. Every time I saw them, they were attached to tree limbs, but only underwater. They had the texture, or feel, of a football. They looked hollow. Does anyone know what they are? I can't find them in any of my nature guides so far. I thought they were some sort of eggsack at first, but I'm not so sure now....