Answer:
The amplitude of a wave can be decreased because of the second law of thermodynamics and/or the differences of the propagation medium.
Step-by-step explanation:
The wave is formed by a transference of kinetic energy between particles in a fluid state. Different medium has different inertia. When a shock wave travels through the water it has a different velocity when the same energy is dissipated as a wave in the air. The lower the inertia of the medium the bigger is the amplitude and vice-versa.
Another way that wave's amplitude might decrease as it moves through a medium is because of energy losses accordingly to the second law of thermodynamics. That means, when a part of a closed system interacts with another part, the energy tends to divide evenly, until the system reaches a thermodynamic balance.