You're building a game, where theres water. Stuff can fall in the water.... and float. People can die, and fall in the water.... bobbing in the surface. Maybe get shot, fall from some height, splash under the water, then slowly bob back to the surface, to dramatically float there... You could go code it, or you could just buy Floating Bodies: get it here: https://www.assetstore.unity3d.com/#!/content/29765