riptide

A current flowing from nearshore waters back toward the open sea, gentle in some cases, violent in others. While riptides can contribute directly to good waves, they are frequently dangerous to swimmers and novice surfers. "Riptide" is something of a misnomer, as it's influenced but not caused by tide. A riptide is also known as rip current, or run-out, or (another misnomer) undertow.

Riptides are generally formed when waves break consistently over a shallow sandbar or reef, and the incoming...