If you are going to make a trip to San Marcos, please consider leaving your pets at home. The San Marcos River in particular is a fragile ecosystem that is home to several endangered species. For example, there is a blind salamander which is delicious to pets, but in danger and not to be shared with them.
Also included in the city ordinances are for no alcohol in or around the river. I guess the mixture of alcohol and college students are particularly bad for these endangered animals as well.
As great travelers, I would highly recommend the phrase... take only pictures, and leave only footprints when traveling to San Marcos.
One of the popular activities in San Marcos is to swim and/or float on the San Marcos River. While in the river, you will likely want to be careful in certain areas, as the undertow can get very strong. The area of the small waterfalls next to Saltgrass Steakhouse is particularly dangerous, so make sure to be careful while there.
Just remember, while swimming in the river can be fun, be careful and watch where you decide to swim.