The ornamental design for a watch, as shown and described.
FIG. 1 is a perspective view of the top and left side of a watch showing my new design.
FIG. 2 is a top plan view thereof.
FIG. 3 is a left side view thereof.
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a right side view thereof.
The broken-line disclosure in the figures illustrate an example of intended environment for illustrative purposes only and forms no part of the claimed design.