A Very Basic Tutorial - Water.
There are other ways, but this is basically what needs to be done for water plane or the Water plug-in.
You can use a water plane, but that will mean limited water interaction. It is just a flat mesh with a water material applied & no collision.
(Optional) Add a physicsvolume, enable water volume, set fluid friction. Sometimes the effect is barely noticeable, but it is applied. This is a good option when using a "water plane mesh"
(Optional) Add a postprocess underwater material & place the postprocess as low into the water as possible so the camera isn't affected above ground.
The Post process is triggered when the camera position hits the Post process volume.
You are better off using UE's built in system for this. It has more water interaction options. You could also use a 3rd party option. Make sure to enable the plug in if using UE's built in system.
Changing the Postprocess for UE's built in water is done here. You can also change all the PP settings for underwater just a little lower down the settings page.
This is how you change the water colour.