Terrain Question?

How do i make it so i can put units on water? ie Buildings and Footmen?

Only if the unit's movement type allows it, or in the case of buildings, the pathing map.

I'm not to sure I understand exactly what you are asking, regardless I will attempt to assist you.
With my previous experience in WE, I have always been able to place 'footmen' in the water, or do you mean above the water, as if floating? If thats the case then you would need to modify 'Unit Hieght' and it does not have to necessarily be a 'Hover' or 'Flying' type unit, but just increased movement hieght so it can float above the water. As Elven Ronin pointed out though, the units movement type should allow it for most creatures or non buildings, but just double check under the 'Unit Editor' if you remain unsure.

With buildings however you will need to edit the 'Pathing Map' to allow the building to be placed in water. Alternatively, under the Unit Editor, enlarge the 'Scaling Value' of the building, then raise a small piece of land above the water and place the building on top of that, it should cover the small piece of land and appear as if in water. (If the building was created large enough).

Hope that wasn't too confusing, as it's easier to do, then for me to explain ^_^ maybe i'm just a bad teacher, anyway, I hoped that helped.

Or just make the unit's "Movement - Type" to be Amphibious.

Make building's "Pathing - Placement Requires" to none. If you want the building to float on water as well, make the "Movement - Type" to be Amphibious, like the units.