Whitewater is often loud and presents a challenge when we need to communicate. Many boaters carry a safety whistle so they can warn others about a very dangerous situation. This is done with 3 whistle blasts - the universal sign of danger. A single whistle blast is commonly used to catch a person's (or group) attention. Try not to overuse the whistle as this reduces its effectiveness (like the boy who cried wolf too often).

When running a difficult river, the lead boat (scout) runs the drops first and sometimes needs to quickly relay guidance back to the rest of the team. Ideally, this is done with paddle signals (or sometimes hand signals). Using the paddle is preferred since it is large and easy to see. When signaling others on which route to take, always point in the direction you want them to go - never towards the obstacle.

Here are the common signals:

The GO Signal
The Stop Signal
The Direction Signal
I'm OK Signal

There are a number of other useful signals as described in this useful article: River Signals.
This is another excellent site for the common signals: Common River Signals.