I wouldn't use it on a kayak where the gunnels get twisted as in traditional Greenland kayak construction.

Its OK for everything except ribs and the hoop. I would not go less than 1/2" x 1/2" for chines on a multi-chine kayak and lash over the chines rather than having drilled holes, gunnels should be 7/8" thick minimum.

Keep notches for lashings as shallow as possible. Don't let anyone sit in your kayak except when it is supported in the water. For average use red cedar is OK - for a heavy person and rough use then I'd use a stronger wood.