The problem with diffuse energy sources such as waves and gentle currents is that the more diffuse the source, the bigger the machine you need to capture a useful amount of energy. Same as sailing - small sails in heavy wind, big sails in light wind. In the energy biz, bigger equipment is almost always more expensive, and that means a poor return on investment. There's a huge amount of energy freely available in the world, but most is spread out too much to capture efficiently. Even the more concentrated sources such as sunlight in the southwest and winds in some of the northwestern states are not constant so they can only supplement a baseline generating capacity. Sometimes this can be advantageous such as in the southwest where peak sun corresponds to peak air conditioning, but in other cases the energy would have to be stored so it can be released when needed, which is another big expense. All of these things are doable, at a cost. There's a lot of work going on to figure out how to make just about any energy source you can think of cost effective.

Unfortunately, the propellor on the roof of a car makes no more sense than mounting a fan on the transom pointed forward at the sails to drive a sailboat - the push backwards from the fan would balance the push forward from the sails.