You need to set the specular to 1 and the roughness to 0 (or values that appeal to you)
also you will need to have a Sphere/Box Reflection Capture actor covering your water or you activate planar reflections in the Project settings and in the material, but that costs alot of calculation.
the essencial steps for a water material are:
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Select the Material node with the Inputs to see the settings in the details tab
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Set the Blend mode (material panel) to translucent
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Set the Lighting mode (translucency panel) to surface TranslucencyVolume or Surface Forwardshading - this is needed to activate specular
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On the specular and opacity channel you can use a Fresenel node
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If you like you can put a single value of 1.3333… into the refraction input (it’s the IOR for water)
Some people also set metallic to 1 wich is not physical correct, but do whatever suits the look 
For the Opacity it would actually need the distance from the surface to the object behind it. This is more complex to get, but you should be fine with a fresnel node.
edit: if you want to use the refraction and get strange looking streching artifacts in the water, try to set the refraction from Index Of Refraction to Pixel Normal Offset, it’s less accurate but also less ugly ^^.