There is a trade off when using a pulley. I am fairly certain you realize this fact. The limit of course is the vertical displacement of the "ball" trading force over distance. You also no doubt understand the power plant has to weigh more than the rubber ball. A cubic meter of water is 272 gallons. To get close to the amount of energy to pressurize the seawater to 900 psi you ball has to be at least a cubic meter. By my accounting the weight is roughly 2,176 pounds. That is just make make it neutrally buoyant. More weight will need to be added just to keep it on the ground. Peak to peak wave form is on average every 9 sec so I am having a hard time understanding how you expect to get 400 liters out of this system?