Whistles

 Canyons can be really loud when there's a waterfall crashing down next to you, so communication can be seriously compromised. A good quality whistle / Survival Flute that can be heard over the roar of a waterfall or where you have a really long abseil and can't hear each other properly can help with communications.
Whistles for outdoor use are piercingly loud. Avoid the use of whistles with moving parts as they can clog and jam up when soaking wet.

As there are a number of different variations and interpretations of whistle signals and communications in general, you should confirm communications to used prior to starting your canyoning trip.
Suggested Whistle Signals based on Grant Prattley's Canyoning Technical Manual:
1 blast = Stop
2 blasts = OK
3 blasts = Down
4 blasts = Up
Continuous = Help

For a system for canyoning that is recommended by the FIC, see: Canyoning Knowledge Base / International whistle communication system in canyoning

If you can see each other then hand signals are a good alternative and you can always combine hand and whistle signals.