The beautiful Anahola Beach Park is one of Kauai's few life guarded beaches and one that's very popular among locals. The beach at Anahola bay extends for over a mile, but it's the beach park at the eastern most end that has good swimming thanks to a reef that protects from waves. Despite the reef, there isn't snorkeling here - the visibility is poor and there aren't many fish. The beach is usually quiet during weekdays.
The northern sections of Anahola Bay past the river offer swimming, snorkeling and surfing but strong currents are always present. Unfortunately Anahola Beach Park has a reputation as not being too welcoming to tourists. This should not disuade you from visiting especially on a weekday, but given how many beaches there are to explore on Kauai, there is also no need to go out of your way to visit this beach.