Most mammals of any size will not float immediately after death in water. Usually anywhere between 7-14 days (depending on water temp, etc) if the carcass hasn't been eaten it'll float up, but not initially.
Most birds will float initially just due to their innate buoyancy from the feathers. Mammals, not so much.
Last edited by dlowrance; 05/04/18 03:01 PM. Reason: missed a word