I can tell you from experience that kayaking is a great all-around abs exercise because it is really sort of a mix of boxing and horseback riding in form. You are constantly twisting inside the boat:
Although non-paddlers often think its an all-arm exercise, what actually happens is the legs/knees act as springs between the hips and feet, storing and transferring potential into kinetic energy. At the same time, the arms in proper motion are punching in succession (not pulling which is inefficient, hence tiring)
Long-story short is, your core is the connecting structure that makes this all happen. So...constant all-angle workout of the abs.
I don't have access to a river right now, so I make do by visualizing and doing weighted twists and stretches that somewhat mimic paddling: bar twists, the torso rotator machine, punches and twisting while kneeling is a zaizen pose, etc. Not the same, but you get the idea.
You could always go paddling. Whitewater is more effective for an all-angle workout, but sea kayaking also works. Gym stuff is more like sea.
Hope this helps.