The biggest problem for scullers on rough water is their tendency to stiffen their upper body, arms and hands. Once this happens, the oars can no longer provide stability during the stroke, and every bit of roughness in the water is transmitted to the body of the sculler, compounding the stability problem. The strongest scullers are the most likely to fight the water, particularly by breaking at the elbow shortly after the catch. The first advice for rough water rowing is to relax and learn to work with the water. The same relaxed shoulders and light hands that allow good handling of a shell in flat water will smooth out the stroke and stabilize the shell in rough water by allowing the oars to stay with the water during the stroke.
If the boat gets caught in an eddy or tossed suddenly by large chop, and you feel in danger of capsizing, pause briefly with the blades feathered. This will stabilize the shell. Wait, relaxed, to find out how the shell will move before you take the next stroke. If the rough patch is small - just a couple of waves, a wake - you can ride it out with your blades feathered and on the water. If the rough water covers a large area that you must row out of, start with easy strokes, adding power only if and when you are confident of your balance. As you become more comfortable rowing in rough conditions, you will find yourself able to row comfortably in larger and larger chop.
The same technique should be used if the bow of your shell noses into a wave. You will feel your shell stall and may see the stern slide to one side, especially if your skeg is not big enough or mounted too far aft to stay submerged. To give the shell time to free itself, take the power out of that stroke and move your weight into the stern of the boat. In a few seconds the bow will be free and you can pick up the stroke again. When rowing in the same direction as good sized chop or waves, it is best to angle the bow slightly to one side to avoid nosing into waves. When in doubt, back off!
Never put a death grip on the oars to handle rough water. This can be the single, biggest problem when rowing in rough water! Keep as light a grip as you can by easing the pressure of your thumbs on the ends of the oar handles and relaxing your grip so that your palm is not in contact with the oar handle. This will let your blades stay at the proper depth in the water during the stroke, thus helping to balance you. Loosening your grip also keeps the blood flowing, lessens the possibility of forearm cramps, and allows you to keep your feel of the water through your touch on the oars.
Relaxed shoulders are necessary whenever you row. In rough water, they are even more important. Loose shoulders allow the arms to act as shock absorbers in rough water, swinging to whatever position the blades require to stay at their proper depth during the stroke and the proper height during the recovery. If your blade hits a wave top on the recovery, a relaxed shoulder will allow the oar to bounce up and be repositioned at the proper recovery height, all without passing the shock through to your body. Think of the oars as being attached to the body at the shoulder; your arms are merely extensions of the oars and should respond to whatever movement the oars require to produce the correct positions during the stroke. So allow your arms to always swing at the shoulder.
Keeping your knees low, by using 1/4, 1/2 or 3/4 slide, lowers your center of gravity and helps balance the shell. When the water is rough, you need shorter, more frequent strokes and steady, smooth power. The slightly higher stroke rate will make up for the loss of run (glide between strokes)that you will experience in rough water. Experiment with the length of your stroke when rowing long pieces in rough water. When you find the right stroke length for the conditions - both the size of the waves and the wave period (the space between wave crests) - the shell will settle down and handle more smoothly rather than bucking through the waves and stopping dead as it hits each wave.
In extremely rough water, stop your hands about 3 or 4 inches away from your ribcage at the finish of the stroke. This will allow more room to drop your hands and release the blades from the water. If you finish with your hands close to your ribcage, as you do in flat water, you risk jamming an oar handle into your hip if the shell is suddenly slapped to one side just as you are releasing the oars from the water.