Rip currents are the most consistent hazard on surf beaches, and they are not random. They form where water piled onto a beach finds the easiest route back out.
Breaking waves push water shoreward
Every wave that breaks carries water up the beach, and that water raises the level near the shore slightly above the level further out.
That difference has to resolve, and the water returns seaward through whatever path offers least resistance along that stretch of coast.
Where the return is spread evenly the flow is weak, but where it is concentrated into a narrow channel the result is a fast, focused current.
Sandbars create the channels
Waves shape sand into bars running roughly parallel to the shore, and those bars are rarely continuous. Gaps develop where flow has already cut through.
Water trapped between the bar and the beach runs along the shore until it reaches a gap, then accelerates seaward through it.
Because the current keeps the gap scoured clear, a rip tends to persist in the same place for as long as the bar system holds.
Structures produce permanent rips
Groynes, piers, jetties and rocky outcrops all provide a hard edge along which returning water can move without obstruction.
Rips beside such structures are among the most reliable on any beach, which is why lifeguards routinely site the swimming zone away from them.
The deceptive part is that water beside a pier often looks calmer, since a rip suppresses breaking waves along its path.
What a rip looks like from the beach
The usual signs are a gap in the line of breaking waves, a channel of darker or choppier water, and foam or sediment moving steadily seaward.
Because the flat area looks inviting compared with the breaking water either side, swimmers are drawn towards exactly the wrong place.
Spending a few minutes looking at the water before entering is what allows these features to be seen, and they become obvious with a little practice.
Why the standard advice takes the form it does
A rip carries a swimmer seaward, not under, and it weakens beyond the breaking waves rather than continuing indefinitely.
Advice to avoid swimming directly against it and instead to signal for help, float, or move parallel to the shore follows from that, since the current is narrow and fighting it exhausts a swimmer quickly.
Guidance differs slightly between countries and services, so the instructions posted at the beach and given by its lifeguards are the ones to follow.