I think you will find that for diffuse light to work, the surfaces need to be in extremely close. The distance involved between two glass surfaces in contact is really small. But, the distance between the surface of water and the glass under it is too thick for diffuse light to form fringes. (at least with all the experiments I tried). You can get water thin enough to see fringes with diffuse light if the glass surface is simply wet, but capillary action of the water that thin will cause it to bead up and deform thus producing very erratic, therefore unreliable, fringes. With collimated light, the distance between the two surfaces can be much greater, therefore it works well with water. Don't forget that the fringes with water are going to be dimmer than they are between two glass plates because of the different indexes of refraction (reflection??) involved.

If you succeed in getting fringes with water and diffuse light, I would be interested to hear it.

Dick Parker