When I create a new material and assign fx/water as its shader,
what do I need to assign as Internal Reflection?
The water4example material has a cubemap? assigned from it, but when I click on the example cubemap I do not see its source in the project folder.
Without this my water has this turqouise solid look.
the Cubemap reflections for the water4example material are generated internally by render textures and cameras which are generated when you apply the ‘Water’ script to generate real-time reflections. The quick way to apply this once you have imported the pro water package is to simply drag the Daylight/Nightime Water material from: Water(Pro Only)\Sources\Materials onto your plane/mesh.
Then from Water(Pro Only)\Sources\scripts drag the Water.cs script onto your mesh and the cubemap will be created for you.
To do this manually to your own material you would need to set up 6 cameras camera in the relevant position and directions (up, down, left etc) and assign 6 render textures to them. Then you would apply those 6 render textures to the faces of a cubemap and assign that cubemap to your material internal reflections/refractions, but again there are elements of the script that would do this for you with relevant offsets/parameters so it would be much easier to use it.
For further tips check out: Tips for water in Unity | Unity Blog
the water base generates it for you. So just drag the newly created water material into that script and let it do its magic.