The best product for kayaking is called the FREEDOM7.
Most people place the trailing antenna through the scupper hole during fishing or have it hanging over the side with the Velcro pouch attached to the chair. You need to ensure both electrodes are fully submerged to create the protective field. The first electrode is 250 mm away from the electronic housing as can be seen in the attached diagram.
In general ocean kayakers use the FREEDOM7 to protect their catch and themselves during fishing, and tend not to use the device when paddling long distances as the trailing antenna does create some drag.