Carbon is probably overkill.
Most sailing kayaks use either aluminum or wooden masts. Most of the aluminum ones have no taper and are just chunks of tubing and work fine. If you are planning to use a sail cut like a windsurfer's with a lot of luff curve, you would probably need a tapered mast. You can even buy mast tubing for kayak sails from Chesapeake Light Craft as I remember. Carbon is probably overkill.