The reality is that a rapid can be noisier than a Stones concert, and vision from a gyrating kayak can be quickly distracted by current and obstruction. That's why the American Whitewater Affiliation developed a set of quickly recognizable signals that every river paddler should know and use.
STOP: The paddler ahead of you has seen some potential hazard, and is telling you not to proceed. This is not an invitation to debate the nature of the potential hazard but an absolute "Eddy out or get off the river!"
The signal: form a horizontal bar with your paddle or your outstretched arms. Keeping the paddle shaft horizontal, you can (but don't have to) pump it up and down to attract attention. If you're using arms, attract attention with a flapping or flying motion. If a paddler ahead of you makes this warning, pass it along to the paddlers coming behind you.
HELP/EMERGENCY: Go to the aid of the signaler as quickly as you can.
The signal: there are three possible parts, and you can use them separately or in unison to attract the most attention. If you have a whistle (and you should, because there's no way to holler over the rumble of even a moderate rapid), blow three long blasts. Just like with exercise, you can do a few reps. Hold a helmet or life vest over your head, as high as you can, and wave it in a tight circular pattern. Hold your paddle vertically, waving it in a tight, fast circle. Your best bet is to make the sound and motion together.
ALL-CLEAR: Come ahead (and you can take a moment to swallow your heart back out of your throat if it had been preceded by a STOP). When the signal is given by itself, either from a boater in the river or a paddler on the shore, it means to proceed down the center.
The signal: form a vertical bar with your paddle, with the paddle blade at right angle to the paddlers upstream for maximum visibility, or hold one arm vertically above your head. To signal the direction of a preferred course or direction through a drop, lower the previously vertical "all clear" paddle or arm by about 45 degrees toward the side of the river with the preferred route. Remember, point toward the CLEAR passage and never toward the obstacle or obstruction you wish to avoid.
The first boat down the river is normally the most experienced boater or the person with the most knowledge of a particular run. No one else in the group should ever pass this lead boat. The sweep boat (the last boat) is normally the second most experienced boater and carries first aid and rescue equipment since he or she will always be upstream of an accident and can get to the site quickly. No other boat should ever fall behind the sweep boat. The rule on the river is to never lose sight of the boat behind you. If you do lose sight, stop and wait until that boat again links up with you. Using this simple discipline, every member of the group is in touch with every other member.