The correct answer is option 2: Increases.

When a tsunami wave approaches the coast, the water depth typically increases. This is because a tsunami is a series of long-wavelength waves generated by a sudden displacement of water, usually caused by an undersea earthquake or volcanic eruption. As the tsunami waves travel towards the coast, they carry a large amount of energy and cause the water in front of them to rise.

As a result, the water depth increases significantly as the tsunami waves approach the shore. This increase in water depth can lead to devastating and destructive effects on coastal areas, as the higher water level can cause flooding, damage buildings, and sweep away objects and people in its path.

It is important to note that tsunamis are different from regular ocean waves. Unlike regular waves that only cause the surface water to move up and down, tsunamis involve the movement of the entire water column. This is why they can cause such a significant increase in water depth as they approach the coast.