The ornamental design for a kayak, as shown and described.
FIG. 1 is a prospective view of my kayak design taken from the right; it is understood that the prospective view taken from the left is a mirror image thereof
FIG. 2 is a is a top plan view of my new design; and,
FIG. 3 is a side elevation view of my new design.
The broken lines are for illustrative purposes only and form no part of the claimed design.