[zakary]

You’re right that nothing is unlimited. Luckily in this case, the energy of waves and tides is given to the oceans mostly by the gravity of the moon pulling the water as it passes overhead. Also a little bit by the wind which is fed by solar heat energy. The moon is slowly moving away from earth and eventually, in hundred of millions of years, it will impart significantly less energy into the tides.

The energy that the moon gives the tides is essentially the same as how the sun gives energy to the ground with light. That is to say: If we don’t collect it, it just gets turned into another kind of energy that is absorbed by the environment. For tides and waves that would be mostly heat, and a little sound. And most of that heat would eventually be radiated back out into space. So suffice it to say, while there is a finite pool of energy stored in the tides, it is so massive we could never make a difference, and it gets recharged everyday by the moon.