Go to the generic browser and find yourself a nice plane. Package UN_SimpleMeshes (loaded by default), has one.
Add the plane to the level and position it correctly. You may need to scale it up a lot to make it fit the area.
In its properties, expand the section Collision and set CollisionType to NoCollision.
Next, expand the section StaticMeshActor, then the section Lighting, and disable everything.
Leave the properties window open and return to the generic browser.
Find yourself a neat water material. The packages UN_Liquid and UN_Liquid2 would be a good place to start. I personally used material UN_Liquid.SM.Materials.M_UN_Liquid_SM_DistortionRiver_01 for this tutorial, even though it is a one-sided material, it is kind of a problem as you will notice later. It is also possible to create your own water materials, but this is quite a complex process, explained in detail in my two water material tutorials. So please refer to those if you plan on doing so...
Argentina
Australia
Austria
Belgium
Brazil
Bulgaria
Canada
Chile
Colombia
Cyprus
Czechia
Denmark
Ecuador
Egypt
Estonia
Finland
France
Germany
Great Britain
Greece
Hungary
India
Indonesia
Ireland
Italy
Japan
Latvia
Lithuania
Luxembourg
Malaysia
Malta
Mexico
Netherlands
New Zealand
Norway
Philippines
Poland
Portugal
Romania
Russia
Singapore
Slovakia
Slovenia
South Africa
South Korea
Spain
Sweden
Switzerland
Taiwan
Thailand
Turkey
Ukraine
United States