Yamaha unveils first prototype hydrogen outboard

It takes a considerable amount of energy to break the water molecule in order to get the hydrogen.  As others have stated it takes way more energy to produce the hydrogen than the amount of energy that is produced by the hydrogen.  In other words a there is a large net loss of energy.  

      Over coming this problem will be very difficult but not impossible.  It is impossible to get more energy from the hydrogen than it takes to produce the hydrogen, and this fact will not change.  The solution lies in finding a way to overcome the problem by using a less expensive non polluting source of energy to produce the hydrogen.  If for instance solar energy is used it doesn't matter how inefficient the energy conversion is because the source of energy is basically never ending.

       We are a long way from being able to produce hydrogen, from solar energy in large enough quantities to run a fraction of the outboard motors used in the US, but the Wright Brothers airplane didn't break the sound barrier either.  You have to start somewhere, it may end up not being feasible, or it may be revolutionary, only time will tell.