A spare paddle, I put a little 2' wooden paddle I got for one of the kids in the back of the yak. Pump or at least a gallon milk jug cut to make a bailer. I'm not familiar with the hydrostar system but being visible to boats is a must whether it's waving your paddle during the day or hitting them with a white light at night you need to make sure they see you. Not a safety item but I've always used one of those synthetic chamois when canoeing and kayaking works great for keeping the bottom puddle free.
Keep the knife, whistle and light on your person not just somewhere in the boat.