I want to put in a pre-formed water garden.
Should it be in full sun, shade, or half sun/shade
You are welcome here but there is a ponds forum. If the shade is from trees, you will have too many leaves. Even in summer in dry weather my trees are shedding leaves. If the shade is from a building or lattice it would be OK depending on what plants you want. I think water lilies want sun but some other plants may tolerate shade.
I looked this up on the pond forum and saw that you posted there also. It is true a small pond in the sun will get too hot. I have black stock tanks and in the sun they get too hot. My pond in the ground is covered in part by water lily leaves and although I consider it small, it is bigger than a pre-formed pond liner. It does not get too hot in the sun. The part about the leaves is still a consideration. After giving it more thought, I realize I have the worst trees. Walnuts and river birch. The walnut leaves are kind of nasty and turn the water brown. The river birch sheds leaves at the first sign of dry weather. So You should consider what kind of trees you have. Everyone has leaves in the fall. You can deal with that.