The best fish finders for a kayak will be built to survive constant exposure to the elements. Fish finders with high IPX ratings are waterproof, dustproof and damage resistant. Look for plugs that are protected from moisture and dirt. Mount the fish finder display in a location that is most protected from swinging paddle blades. The weak link is the connection to the battery. Use heat-shrink terminal connectors and treat all electrical connections with dielectric grease.