I just got back from Austin and I can highly recommend renting a boat if you want to enjoy a day on the water! It was one of the highlights of my trip. We rented a boat to cruise around Lady Bird Lake and it was absolutely wonderful. The water was clear, the views were stunning and there was so much greenery and nature around. What I especially loved was that we could stop at the little islands to swim and just relax. Boat rentals are easy to find through local companies - there are plenty of options online and you can often choose between a captained boat or a self-guided boat. We chose the captained option so we didn't have to worry about navigation and just enjoyed the experience. This is also convenient if you are not familiar with the area.