This is one of the major attraction of the area, either you just stroll along or decide to do something more wet!! I was there in november but apparently tube renting is very popular during the summer; have a look to the following link for the details
The river doesn't look more than 1-2 meter deep and the water doesn't seem too clear. You could imagine how surprised I was to see two persons scuba-diving with their full equipment!! Either I have missed something or some people are really crazy...
True that there are over 100 observable species of underwater plants and many species of aquatic life, that's probably the reason!
The water temparature is said to be around 22C all year round but I prefered not to try ;-) There is a nice walking path along the river and if you are hungry, pick nick area is also available in the park.
The San Marcos River is a nationally recognised natural treasure. Recently, National Geographic magazine featured the San Marcos River in a special issue on fresh water systems and the river has been the subject of many books and videos.
San Marcos River receives much of its water from the environmentally at-risk Edwards Aquifer.
Float the River! A trip to San Marcos would not be complete without a float down the Guadalupe River.
This is a popular activity during the summer months because it's about the only way to cool off in the central Texas heat. Don't forget to rent a tube for your cooler because a trip down the river just wouldn't be the same without beer!