What kind of soft cooler should I get for kayaking?

Get a rotomolded soft cooler with a waterproof zipper, like the Yeti Hopper or Engel Deep Blue. They’re expensive, but cheaper options won’t stay leakproof after a few trips because they use standard zippers that fail. A proper waterproof zipper and rotomolded construction keep water out even if you flip your kayak or drop the cooler.