I teach Wilderness First Aid and so my kit is usually a bit more than most carry with them. That being said, I can't think of a single time in the past 3 years that something from my kit hasn't been used on a river or in camp.
I started with a zip pouch kit and replaced all the useless bandages in it with water proof ones. I then replaced the stupid plastic tweezers w/ metal ones w/ a sharp point. After that I just added a bunch of stuff that gets used frequently.
In addition to the basic stuff that came in the kit (measuring about 8" x 6" x 2") I added:
-a plastic sryinge to irrigate wounds and clean debris out of eyes
-eye drops (especially useful in dusty areas or for unexpected visits)
-duct tape, vet tape and moleskin
-mini LED light
-electrolyte mix
-honey packet (for diabetics)
-lots of motrin
-maxipad for blood absorption
-space blanket
To the small drybag I keep the kit in I also added an ace bandage, SAM splint, tampon, waterproof matches, instant ice pack and sunscreen.
Not super compact (ie for a kayak) but it fits nicely into my drybag of goodies on the raft or in the back of an IK.
With all that- I have to say the most useful piece of equipment is your BRAIN. Having a lot of cool stuff in a kit will be useless if you don't know how to use it, where it is or how to access it.