The problem is not the size of the ship, but the fact that you will use just the same amount of energy ( at best ) to pump the water up as you will gain back when you let it drop down. Furthermore, since the ship is floating in water, any water you take in would cause the ship to sink a little further into the ocean and you would lose all the energy into the surrounding waves.(Original post by Kernel)
Are there any ships that use hydroelectric power, generated while they sail? Is it possible, or are ships too small?
If you are looking for renewable energy to use for ship propulsion, wind, solar, wave or tidal power is more likely to be useful. Two of these have been used for millenniums. Care to guess which two? :P
Find out the top ten here