Water is very simple to add. Simply create a gameobject and add a collider box component. In the tags section at the top of the gameobject add a tag called 'water.'

Make sure the collider has 'Is Trigger' checked and 'Static' unchecked.

The dimensions of the collider box will be where players are in water. Make sure the top of the collider is the same height as the top of the face with the water material.