I recently spent a week on Lake Superior at the Apostle Islands. This shot came from something I worked on last time I was there a couple of years ago - attach a tripod mounted camera to the kayak and shoot with long-ish shutter speeds. Instead of paddling while doing this, I tried rocking the boat back and forth (while not in it) to blur the sides for the 1/4 sec exposure. This kept the horizon fairly sharp in the middle and made the effect stronger at the edges.
Coolpix 5000 with wide lens adapter (about 19mm). Only Photoshop work was cloning out a couple of distracting elements on the boat - like the tips of the tripod legs that I didn't realize were in the shot!