Dangerous currents are a serious hazard for swimmers and boaters alike. An UNDERTOW is a strong current of water that runs beneath the surface of the ocean, often caused by waves crashing against the shore. A riptide is a powerful, narrow current of water that flows away from the shore, usually caused by the tide. A RIP is a strong, localized current of water that flows away from the shore, usually caused by the tide. All three of these dangerous currents can be hazardous to swimmers and boaters, and should be avoided.