I'm assuming this is after the fact of fishing supplies for a fishing kayak (bait, tackle, rods/reels), and assuming a paddle and PFD is already packed.
I must have plenty of water (and maybe snacks if it is a long day on the water), a good knife or multitool, extra paracord or rope, first aid kit, cell phone, sunglasses, hat, bug spray, and sunscreen/or longsleeve fishing shirt and neck gaitor/Buff.