High and low tides are caused by Moon gravity attracting water. Now there's friction, waves cause erosion, their energy is used in power plants yet the tides work for millions of years and are perceived as a free source of energy.
Now that's impossible - energy can't just appear out of nowhere.
What is the energy source for the high and low tides?
To essentially quote http://en.wikipedia.org/wiki/Tide:
Energy of the Earth is not conserved while energy of the Earth-Moon system must exist. Energy from bodies of water are diminished (by about 3.75 TeraWatts) where about 98% of this energy loss is due to marine tidal movement.
Because energy is lost in the water, this imposes a torque on the Earth which changes its Rotational Kinetic Energy ($KE_{rot} = \frac 12 I \omega^2)$. Because angular momentum is also conserved in the Earth-Moon system, that gradually transfers angular momentum to its orbit (By conservation of angular momentum to the Moon's orbit ($L = I \omega$, Earth spins less, Moon gets pushed further away) The equal and opposite torque on the Earth reduces its rotational velocity, thus lengthening the day by about 2 hours per 600 million years.
$E = \frac {1}{2}( I_{earth} \omega_{earth}^2 + I_{moon} \omega_{moon}^2) = constant$
A torque on the earth causes its angular momentum to decrease
$\mathscr{T} = \frac{dL}{dt} = \frac{ d{I \omega}}{dt}$
But Angular momentum is conserved, so the moon must rotate faster around the earth
$L = I_{earth} \omega_{earth} + I_{moon} \omega_{moon} = constant$
