There are several situations that cause strong water currents. First, high surf will cause strong currents. The breaking surf washes up on the beach and has to find it's way back to the Gulf or Atlantic. This may cause what is known as a "rip" current, a fast moving stream of water moving from the shore out past the breakers. If you get caught in it, don't try to fight it, it won't take you to Mexico. Just swim out of it by swimming parallel to shore. Then let the waves carry you back in. I've been swimming and surfing on the Gulf coast all my life and I've never felt threatened, even though I've been knocked down and pulled off my feet numerous times in high surf.
In the photo above, you can see the perfect setup for a rip current. At the time the photo was taken, the tide was low and the surf was small, so the rip channel was safe. As the water gets deeper and the surf comes up, the rip channel becomes dangerous.
You should understand that there is usually another type of water current taking place at the beach. It is called the "longshore current." It is also referred to as the "drift." Most people notice this current when they enter the water and play for a while, then look up and notice that they have unknowingly drifted far from their beach towel. It is important to be aware of which way the longshore current is moving. If you get caught in a rip and try to swim out of it by swimming parallel to the beach, you don't want to swim against the longshore current or it will just carry you right back into the rip.
Rip currents have nothing to do with the tides. The term "Rip Tide" is not correct for referring to this type of current in Florida.
What Causes Rip Currents?
Rip Currents are not mysterious, unpredictable phenomena, like the "Bermuda Triangle," threatening to swallow up innocent swimmers randomly. Rip currents are created when the surf moves large volumes of water onto the beach. All of that water has to find a way back to the sea. The water piles up until it develops such a volume that it creates a "reverse current" and flows back out to sea. If the surf is up, there will be rip currents. You can learn to recognize them once you know what to look for.
Rip currents sometimes appear as turbulent water that seems to interrupt the pattern of the breaking waves. Rips are often a slightly different color than the surrounding water because they are carrying a lot of sand. Other times, as you'll see on the excellent video below, a rip can actually appear calmer than the surrounding surf.
Other Strong Water Currents
High winds will also cause a strong current. This type of current is usually a longshore current and is very common, even in very small surf. It can carry you down the beach quite a distance before you realize you've drifted so far. It is more of a nuisance than anything else. You go in the water at point A and get out at point B and realize you don't recognize your beach towel anywhere.
A large volume of water moving through a narrow channel will also cause a very strong current. This happens daily in most of the passes. A pass is the narrow channel between islands where water flows between the bay and the Gulf. As you can imagine, the current is strongest when the tide is moving either in or out. I do not recommend swimming in or near any of the passes at any time.
Ninety percent of the time there is no surf on the Gulf coast and strong currents only occur at the passes. If you swim in relatively calm waters on the Gulf beach it is highly unlikely you will ever encounter any strong currents. The Atlantic Ocean is a different story.
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Don't underestimate the power of the surf, especially on the Atlantic coast. If you are experienced with surf and are a strong swimmer that's one thing, but so many people come to Florida and swim in surf for the first time and are caught by surprise at the strength of the water. It is especially dangerous for small children. If you are not an experienced surfer, stay out of the surf so you can live to swim another day. The best way to learn how to handle surf is the way the locals do. You start out in very small waves, and gradually, over months or years, move up to larger waves as your experience grows. For Gulf coast surf info, check out http://www.gulfster.com .