Let the waves move up and down. (gravity is free)
fix the plate (with a lip) in position. Use the water pressure as the waves recede to spin generator(s). simple only the generators move, but you really don't even need a plate for this. just a wall along a beach that rests on the bottom and the top is buoyant at a level just beneath the waves.
the outflow should spin the generator(s) quite well.
Mark