The ornamental design for a water sensor and alarm, as shown and described.
FIG. 1 is left side perspective view of a water sensor and alarm incorporating the new design, the opposite side view of the water sensor and alarm being a mirror image thereof;
FIG. 2 is a front perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a rear perspective view thereof; and,
FIG. 5 is a bottom plan view thereof showing a rectangular label area thereon.