Sorry this took me so long, but apparently it is normal to have a gap between the liquid and the Domain; here is a picture from the Blender Manual where you can clearly see the gap:

Look at the little squares (bottom left of each fluid). They are at the corner of the Domains and show the size of each ‘cell’/voxel.
So: increase the size of your Domain, until it fits. Or move the floor up a bit until it touches the water…(I don’t think you care about the sides…)
BTW: applying the scale of your Domain will make the voxel size smaller, maybe your computer can handle that…