How hot does it have to get for kayak plastic to melt?

The kayak will not actually melt, but if you leave it on a hard surface or a roof rack in 100 degree weather the plastic will soften and you can get what is called "oil-canning". This is where the hull develops a flat spot on the bottom or a caved in ridge where it sits on the rack. If you have to leave your kayak on the car rack for a long time on a hot day, lay it upside down so the hull doesn't get deformed OR carry it on its side on J-racks. When you store it in your yard or garage, lean it on it's side which is the strongest dimension of the kayak.

Once a hull is oil-canned it is almost impossible to pop the dent back out and it will reduce the value of the boat and affect the performance. It is the most common cause of damage to plastic kayaks and the most easily preventable.

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Most plastics melt 400 - 500'F. I believe kayaks are typically ABS, which would be closer to 400. No chance of california sun melting the kayak. Now get the kayak off the rack and use it and have fun!!

As they are baked in large ovens at thousands of degrees I wouldn't be too worried - having said that I probably wouldn't pick one up at midday after a week roasting in the sun as they may be a tad hot to touch!