Microorganisms causing diseases that characteristically are waterborne prominently include protozoa and bacteria, many of which are intestinal parasites, or invade the tissues or circulatory system through walls of the digestive tract. Various other waterborne diseases are caused by viruses.
As the Moon revolves around Earth, both objects also progress in orbit around the Sun. After completing one revolution with respect to the stars, the Moon must continue a little farther along its orbit to catch up to the same position it started from relative to the Sun and Earth.
The motion of the Moon around the Earth would continue as it does now, with the field doing nothing to change their relative positions. In fact, the Moon would orbit the Earth in precisely the same way that it would if there were no background uniform gravitational field at all. It follows that what matters is not the average field that the sun exerts on the Earth-Moon system, but how much that field changes from place to place.