San Marcos is a small city, and tends to be very safe. Credit cards are accepted almost everywhere, so it's not necessary to carry large amounts of cash.
Emergency services such as police, fire, and medical can be reached by calling 911.
There are many places to swim in the local rivers. Some are often crowded, some are very secluded. Regardless of where you swim, always use the buddy system, and always have a way to contact emergency services note that cell phones often lose service in the low area immediately about the river, so check the signal at your designated "towel spot" before swimming if you will rely on your cell phone for calling emergency services.
