The lifeguard cabins along the Vice City Beach, bear a sign with the words "RIP Currents" written on it. A rip current, commonly known as a rip tide, is a specific type of water current that can be found near beaches.
It is a strong, localized, and rather narrow current of water. It causes death, if not escaped quickly. However, the phenomenon is not proven to happen in GTA: Vice City, and the sign is there as a joke to say the currents have died, hence the sad face. "RIP" refers both to the rip tides themselves and the common phrase "Rest in Peace."