Unity 5 comes with three different water solutions; they are varied in the level of complexity and hardware requirements. Despite this variety, water in Unity is nothing but aesthetical. It doesn't react to physical objects or add post-processing to the camera when a character gets under it. But, I can't really count that as a negative since rarely do games require water to be more than just a good-looking plane. If you are in need of a more realistic solution for your project, you just may find what you are looking for in the Unity Asset Store.

If you have the Environment package imported, water prefabs can be found under the Standard Assets | Environment | Water, inside of which you can navigate to Water | Water 4 | Water (Basic).

We are going to keep the introduction to Unity Water short and focus on one of the available Water solutions. Find the WaterProDaytime prefab in Standard Assets | Environment | Water | Water | Prefabs and drag it into the scene.

If we take a close...