Make energy out of a capacitor in the sea
Yes, you can convert wave energy to electrical energy in this manner (but it's horribly inefficient &mdash; there are much better ways to extract energy from waves).

Two equations cover what you're proposing. The first is the relationship between charge, capacitance and voltage:

$$Q = C \cdot V$$

The second is the relationship between energy, capacitance and voltage:

$$E = \frac{1}{2}C \cdot V^2$$

Start by putting a charge on a capacitor that's filled with water, and then removing the water. The first equation tells us that if the capacitance goes down by a factor of 81, then the voltage must go up by the same factor, because at this point, the charge can't go anywhere.

But the second equation says that if the capacitor had a certain amount of energy with the water in it, afterward, it has 81 times as much! Capacitance went down by a factor of 81, but \$V^2\$ went up by a factor of 812. E must rise by a factor of 81 in order to keep the equation balanced.

Where did this energy come from? It takes physical work to remove a dielectric from a charged capacitor. In this case, the potential energy of the mass of water within the capacitor was reduced when gravity pulled it out during the low part of the wave. And when the next peak comes along, the energy in the wave pushes the water back up against gravity.

When you put real, physical numbers on the values for C and V that you can realistically work with, you'll soon realize that the power (energy per time) that you can extract is miniscule.