A riptide is an extremely strong, unpredictable current that flows across another. Riptides are especially dangerous for ocean swimmers.
When people talk about riptides, they usually mean the powerful, narrow currents of ocean water near the shore that can be hazardous to swimmers with little experience. Officially, this is incorrect: rip current is the name for this ocean phenomenon, while a riptide is a flow of water in an estuary. As long as you're not worried about being inaccurate, go ahead and use riptide's common meaning — people will know exactly what you mean.