It will depend upon the terms of your insurance policy and your insurance companies own guidelines if the kayaks and four wheelers will be covered or not. It also depends on the state you live in. If they were it would be under your physical damage coverages of collision or comprehensive which normally comes with a deductible.
Since you transport the 4 wheelers and kayaks in your pickup truck on a regular basis you should inform your insurance agent. If your current insurance policy does not cover it, there may be an endorsement that can be made so that these items would be or the agent may be able to suggest a different type of insurance to cover any damages that may occur to these items during the ride in your vehicle.