Lake Austin - We typically wakeboard through the winter, and the water can be decently chilly, down into the low 60s. I would think you would want a wetsuit. Of course, this year Lake Austin temps are up about 10 degrees since they are not releasing much water from Lake Travis (the bottom of Travis is where the cold temps come from on lake Austin). My guess would be that the water temp will drop into the mid-upper 60s if we ever get an extended period of cold weather - which may not happen until January. They typically do not lower the lake until Jan/Feb time frame, but with the drought, they have not dropped the lakes in that last couple of years.
Cycling - weather is hit or miss depending on the cold fronts. Some days are great, where other days are decently cold with a lot of wind. And, there is always the possibility of rain. I would say, however, that at least 2/3 of the days are comfortable. You might get two cold rainy days followed by a few sunny warm days - you never know.
Long and short - you can probably ride and swim but, you may have to delay a workout for a day or two if a front rolls through and you want decent weather.