First Aid Kit
On kayaking courses you should bring a small waterproof first aid kit with you to be carried in your boat. On SRT courses a large group first aid kit is brought by us to the training site.
When rafting every raft carries a first aid kit in the boat. In a small group if every boater has a small first aid kit in their boat then all together you will have a pretty big first aid kit. This also has the advantage that if you still have one boat you still have some first aid equipment.
On some rivers with easy road access I find it an acceptable practice to use canoe tape to fix injuries. As canoe tape can fix 90% of all kayak injuries that you need to mess with (it will even fix your kayak), this can effectively be your first aid kit on easy playboating runs.
On a wilderness river I take a big roll of canoe tape as a repair kit as well as a comprehensive first aid kit including medications which might be needed. Of course somebody on the trip should also be trained to at least Wilderness 1st Responder level in order to be able to treat people in an emergency.