It travels slower in cold, but much faster in water due to the higher density. The reason it travels slower in the cold is because it depends on the motion of molecules. Sound waves propagate when one molecule of bumps into another one which then bumps into another one, and so on. The time between the bumps depends on the density of a medium, which in cold air is lower due to the gas laws. Water has a much higher density than air and so the distance between molecules is a lot smaller. Thus it takes less time for the collisions to occur and the wave fronts advance more quickly.