The claim is quite true and I have seen it many times, but I don't think it's as complicated as some make it sound. It can happen in deep water and without wind. In 1992 my eureka moment came one day while heading east on a motor yacht through the Current Rock passage in the Virgin Islands where I had a birds-eye view of whole effect clearly displayed in front of me. I could see that the current causes the waves in the center to travel a little slower than on the outsides, and that this bends the wave train, and now the waves on the left and right were converging towards the center where they added their energy and height. Depending on the location, the effect can be spread over a large or small area I have illustrations here http://www.3dym.com/waves/waves.htm