Take a look at the photo on this page, and tell me: If you had to give it a one-word label, what would that be?
Non-divers might think something like 'pastels.' To a diver that photo screams current! That's 'current' as in moving water. [You can click on the photo for a larger view.]
Not only are the soft corals in the photo bending like wheat in a windswept field, but all the little fishies are facing in the same direction. That's what little fishies do in a current -- they point their little noses into the current and wiggle their tails for all they are worth, just to stay in place.
Good divers recognize and understand currents in the ocean, and know how to deal with them. The two most common factors relating to the kinds of currents that divers are likely to encounter are wind and tides.
Currents relatively close to the surface can be caused by wind blowing across the water. When wind blows across the surface it pushes a certain amount of water ahead of it. The stronger the wind, and the more open and unsheltered the surface, the stronger the surface current. Most wind-driven surface currents in coastal areas are only a meter or so deep -- a good thing to know. For instance, if there's a wind-driven surface current, a diver will probably have an easier time swimming back to a boat or a shoreline exit point by swimming a couple of meters beneath the surface instead of on the surface.
Some currents are caused by changing tides. The water mass flows toward shore during an incoming tide (called flood flow), and sucks back out to sea during an outgoing tide (called ebb flow).
Currents can be enhanced by underwater topography and the shape of a coastline. Think of the way water in a river or creek flows around rocks and other obstructions. The same thing happens underwater. If there is some relatively large feature in the path of the water mass during a tidal shift, for example, the water will flow around it and the current usually will be stronger and more turbulent closest to the obstruction. The same thing happens where there is a bottleneck -- a place where the flow of water gets squished between two large obstructions.
Currents encountered by divers most often move horizontally, but there are places and times where vertical currents are encountered. Upwellings occur when deeper water rises toward the surface. Downwellings occur in some places, too, but a more common type of downward current a diver may encounter happens where there is a dropoff of some sort. For example, a tidal ebb flow may literally spill over an underwater dropoff, just like a waterfall spills over a cliff on land.
Some currents are gentle, while others rip along with incredible force. Next time I'll tell you about some very interesting experiences we've had dealing with currents while diving.