A circular pattern on the wave face, formed as a swell passes over a raised area on the bottom—usually a large rock or cluster of rocks. Boils are generally a red flag, as they indicate shallow water. They can also be difficult to traverse, as the boil's perimeter will often grab the rail of a surfboard, and because the water within can be unstable.

Boils are greatly affected by tides, and a boiling gyre at low tide might vanish altogether at high tide. In between rides, it is possible to us...