So if its level what does the water do - move somehow or stay put?
You can leave a small gap for any water to drain, or a pointed joint bucket handle style.
But even a flat level surface will only hold a very small amount and only for a short time.
A shed can be set on damp proof plastic bearers, or as above with acurate shed size base.
Like this one i did.
Admittedly this ones plastic.. But it doest suffer from water inside