Surge is the forward-and-back movement of water caused by wave action. It can be a problem near rocks and shallow wrecks, and when approaching an anchored dive boat. Here's what to do:

Step 1: Back Off

Surge, like the waves that cause it, comes in sets of stronger flows, and lulls of weaker ones. If your problem is to approach the boat ladder, back off a few yards and watch the situation until you can read the rhythm of sets and lulls.

Step 2: Don't Fight It

Even moderate surge is stronger than you are. Don't exhaust yourself trying to move forward against it. Instead, hold on to something (a current line, an anchor line, a rock) when the surge tries to carry you backward, and ride it when it carries you forward. Nothing to hold on to? Fin steadily forward just enough to remain stationary. Backward and forward flows are about the same and will cancel out.

• In shallow water. Usually you can avoid surge by going below 10 or 20 feet. (But it depends on the size of the waves.)

• When waves are big. Big surf means surge is not only stronger but reaches deeper.

• At narrow openings. Remember the venturi effect? Surge accelerates in gaps between rocks and through openings in wrecks.

• Around stationary objects. When water has to make a detour around a rock outcrop or a wreck, it goes faster.

Step 3: Timing Is Everything

Wait for a lull (a period of weaker flows) to make your move. Exiting the water? If you time it right, you can ride an inflow to the ladder or rock, hang on during the outflow, and climb out during the slack water before the next inflow. Going through a wreck opening or swim-through? Likewise, ride the inflow to the edge, hang on during the opposite force, then use the lull and inflow to swim through and away from the opening where the surge is strongest. Be cautious on your approach. It's better to fall short of the ladder or opening and have to do it again than to be thrown against it.

When Surge Is Strongest

Why Doesn't the Boat Move?

You and the dive boat are both floating. How come the surge moves you but not the boat? Actually, the boat probably does move, especially if it's small. But the heavier it (or any object) is, the more inertia it has to resist the surge. And it may be big enough to span several waves at once--in which case inflows and outflows cancel each other and it's as immobile as a rock. Plus, if the boat is at anchor, the anchor line restrains the boat's movement.