Watery nature of the cervical mucus at the time of ovulation may be caused by:

Estrogen makes the mucus much thinner and more alkaline, changes that promote the survival and transpo of sperm. Progesterone makes it thick, tenacious, and cellular. The mucus is thinnest at the time of ovulation, and its elasticity, or spinnbarkeit, increases so that by midcycle a drop can be stretched into a long, thin thread that may be 8-12 cm or more in length. Ref: Alford C., Nurudeen S. (2013). Chapter 4. Physiology of Reproduction in Women. In A.H. DeCherney, L. Nathan, N. Laufer, A.S. Roman (Eds),CURRENT Diagnosis & Treatment: Obstetrics & Gynecology, 11e.