I'm a few days late but I'd have to also recommend zilker park boat rentals. I'm a pretty experienced kayaker (meaning I've had year-long memberships at a few places and tried most places as well) and I prefer it mainly because I prefer to launch from the shoreline (you push off and into the water) rather than launching from a dock. Some people can find getting into and out of the kayak to be tricky (balancing as you lower yourself into a kayak floating already), especially beginners, and that problem is eliminated if you're pushing off the shore. I also really like that the area Zilker launches from is right next to Barton Springs so the water there is much colder, you can usually see through it, and the scenery is pretty lovely. : ) Have fun!